Patent Publication Number: US-9839819-B2

Title: Golf club head crown with recess part and step surface

Description:
The present application claims priority on Provisional Patent Application No. 62/207,211 filed in the United States on Aug. 19, 2015, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a golf club head. 
     Description of the Related Art 
     A wood type golf club head having a groove on a crown or a sole thereof has been known. U.S. Pat. Nos. 8,241,144, 8,821,312 and 8,591,351 disclose a head having a stress reducing feature as a groove. U.S. Pat. No. 8,834,289 discloses a head having a flexure as a groove. JP2015-54241 (US2015/0072803) discloses a golf club head in which at least one of a crown portion, a sole portion, and a skirt portion includes a recess-part transition region. 
     SUMMARY OF THE INVENTION 
     From various standpoints, a further improved head has been desired. Inventors of the present application have found a new structure for a crown to be effective from a new standpoint. 
     It is an objective of the present invention to provide a golf club head capable of improving various performances based on a structure of a crown. 
     A preferable golf club head includes a crown, a sole, a face and a hosel. The crown includes a recess part, a back part positioned at a back of the recess part, and a step surface positioned at a front of the back part and positioned above a virtual extension surface of the back part. At least a part of the recess part extends in a toe-heel direction. At least a part of the step surface extends in the toe-heel direction. 
     Preferably, the recess part includes a first side surface positioned on a face side, and a second side surface positioned on a back side. Preferably, the step surface is continuous with the first side surface. 
     Preferably, a distance T between the step surface and the face is equal to or greater than 5 mm. 
     Preferably, a filler is disposed inside the recess part. 
     A preferable golf club includes the head. The golf club has a length of L inches and a real loft of R degrees. The recess part has a depth of D mm. The step surface has a height of H mm. When R/L is defined as X, and D×H is defined as Y, the golf club satisfies the following.
     (1) X is equal to or greater than 0.1 but equal to or less than 0.9.   (2) Y is greater than 0 but equal to or less than 25.   

     Another preferable golf club head includes a crown, a sole, a face and a hosel. The crown has a back part, a step surface positioned at a front of the back part and positioned above a virtual extension surface of the back part, and a recess part extending in a front-back direction. The recess part intersects the step surface. 
     In the present invention, it is possible to obtain a golf club head having various performances improved by a structure of the crown. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a perspective view of a golf club head according to a first embodiment; 
         FIG. 2  shows a plan view of the head in  FIG. 1 ; 
         FIG. 3  shows a front view of the head in  FIG. 1 ; 
         FIG. 4  shows a toe-side view of the head in  FIG. 1 ; 
         FIG. 5  shows a heel-side view of the head in  FIG. 1 ; 
         FIG. 6  shows a cross-sectional view taken along line F 6 -F 6  in  FIG. 2 , and  FIG. 6  is a partial cross-sectional view of a crown; 
         FIG. 7  shows a perspective view of a head according to a second embodiment; 
         FIG. 8  shows a plan view of the head in  FIG. 7 ; 
         FIG. 9  shows a toe-side view of the head in  FIG. 7 ; 
         FIG. 10  shows a heel-side view of the head in  FIG. 7 ; 
         FIG. 11( a )  shows a cross-sectional view taken along line F 11 -F 11  in  FIG. 8 ,  FIG. 11( a )  is a partial cross-sectional view of a crown, and  FIG. 11( b )  shows a cross-sectional view showing a modified embodiment of  FIG. 11( a ) ; 
         FIG. 12  shows a plan view of a head according to a third embodiment; 
         FIG. 13  shows a cross-sectional view taken along line F 13 -F 13  in  FIG. 12 , and  FIG. 13  shows a cross-sectional view of only a crown; 
         FIG. 14  shows a cross-sectional view of a head according to a fourth embodiment, and  FIG. 14  is a partial cross-sectional view of a crown; 
         FIG. 15  shows a cross-sectional view of a head according to a fifth embodiment, and  FIG. 15  is a partial cross-sectional view of a crown; 
         FIG. 16  shows a cross-sectional view of a head according to a sixth embodiment, and  FIG. 16  is a partial cross-sectional view of a crown; 
         FIG. 17  shows a cross-sectional view of a head according to a seventh embodiment, and  FIG. 17  is a partial cross-sectional view of a crown; 
         FIG. 18( a )  shows a plan view of a head according to an eighth embodiment,  FIG. 18( b )  shows a plan view of a head according to a ninth embodiment,  FIG. 18( c )  shows a plan view of a head according to a tenth embodiment; 
         FIG. 19( a )  shows a plan view of a head according to an eleventh embodiment,  FIG. 19( b )  shows a plan view of a head according to a twelfth embodiment, and  FIG. 19( c )  shows a plan view of a head according to a thirteenth embodiment; 
         FIG. 20( a )  shows a plan view of a head according to a fourteenth embodiment,  FIG. 20( b )  shows a plan view of a head according to a fifteenth embodiment, and  FIG. 20( c )  shows a plan view of a head according to a sixteenth embodiment; 
         FIG. 21( a )  shows a plan view of a head according to a seventeenth embodiment,  FIG. 21( b )  shows a plan view of a head according to an eighteenth embodiment, and  FIG. 21( c )  shows a plan view of a head according to a nineteenth embodiment; 
         FIG. 22( a )  shows a plan view of a head according to a twentieth embodiment,  FIG. 22( b )  shows a plan view of a head according to a twenty first embodiment, and  FIG. 22( c )  shows a plan view of a head according to twenty second embodiment; 
         FIG. 23( a )  shows a plan view of a head according to a twenty third embodiment,  FIG. 23( b )  shows a plan view of a head according to a twenty fourth embodiment, and  FIG. 23( c )  shows a plan view of a head according to a twenty fifth embodiment; 
         FIG. 24( a )  shows a plan view of a head according to a twenty sixth embodiment,  FIG. 24( b )  shows a plan view of a head according to a twenty seventh embodiment, and  FIG. 24( c )  shows a plan view of a head according to a twenty eighth embodiment; 
         FIG. 25  shows a perspective view of a head according to a twenty ninth embodiment; 
         FIG. 26  shows a plan view of the head in  FIG. 25 ; 
         FIG. 27  shows a cross-sectional view taken along line F 27 -F 27  in  FIG. 26 , and  FIG. 27  is a partial cross-sectional view of a crown; 
         FIG. 28  shows a perspective view of a head according to a thirtieth embodiment; 
         FIG. 29  shows a plan view of the head in  FIG. 28 ; 
         FIG. 30  shows a front view of the head in  FIG. 28 ; 
         FIG. 31  shows a toe-side view of the head in  FIG. 28 ; 
         FIG. 32  shows a cross-sectional view taken along line F 32 -F 32  in  FIG. 29 ,  FIG. 32  is a partial cross-sectional view of a crown, and  FIG. 32  includes a cross-sectional view of a recess part RE 1  extending in a front-back direction; 
         FIG. 33  shows a club according to an embodiment of the present invention; and 
         FIG. 34  shows a club set according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the present invention will be described in detail according to the preferred embodiments with appropriate references to the accompanying drawings. 
       FIG. 1  is a perspective view of a head h 1 .  FIG. 2  is a plan view of the head h 1 . The plan view is a figure viewed from a crown side.  FIG. 3  is a front view of the head h 1 . The front view is a figure viewed from a face side.  FIG. 4  is a side view of a toe side of the head h 1 .  FIG. 5  is a side view of a heel side of the head h 1 .  FIG. 6  is a cross-sectional view taken along line F 6 -F 6  in  FIG. 2 . 
     The head h 1  is a wood type head. The head h 1  is a so-called fairway wood type. Inside of the head h 1  is hollow. In other words, the head h 1  has a hollow structure. 
     The head h 1  includes a crown c 1 , a sole s 1 , a hosel z 1  and a face f 1 . The crown c 1  extends from an upper edge of the face f 1  toward a back side. The sole s 1  extends from a lower edge of the face f 1  toward the back side. The outer surface of the face f 1  is a hitting face. The hitting face is also referred to as a face surface. 
     The head h 1  further includes a side part d 1 . The side part d 1  extends between the crown c 1  and the sole s 1 . The side part d 1  is also referred to as a skirt. 
     [Definition of Terms] 
     The following terms are defined in the present application. 
     [A Reference State, A Reference Vertical Plane] 
     A state where a head is placed on a horizontal plan H with a prescribed lie angle and a prescribed real loft angle is defined as a reference state (not shown in the drawings). In the reference state, the center axial line of a shaft hole is contained in a plane perpendicular to the horizontal plane H. The perpendicular plane is defined as a reference vertical plane. The prescribed lie angle and real loft angle are appeared, for example, in a product catalog. 
     [Toe-heel Reference Direction] 
     A toe-heel reference direction denotes a direction of an intersection line of the reference vertical plane and the horizontal plane H. 
     [Toe-heel Direction] 
     A toe-heel direction denotes a direction having an angle with respect to the toe-heel reference direction of within ±20°. A preferable toe-heel direction has an angle with respect to the toe-heel reference direction of within ±10°. These angles are measured on a planar view seen from above.  FIG. 2  is an example of the planar view. 
     [Front-back reference direction] 
     A front-back reference direction denotes a direction perpendicular to the toe-heel reference direction and parallel to the horizontal plane H. 
     [Front-back Direction] 
     A front-back direction denotes a direction having an angle with respect to the front-back reference direction of within ±20°. A preferable front-back direction has an angle with respect to the front-back reference direction of within ±10°. These angles are measured at the planar view seen from above. 
     [Up-down Direction] 
     An up-down direction denotes a direction perpendicular to the horizontal plane H. 
     [Planar View] 
     In the reference state, an image projected to a plane parallel to the horizontal plane H is the planar view. The direction of the projection is a direction perpendicular to the horizontal plane H. 
     [Face Center Fc] 
     A face center fc is defined as a centroid of the contour shape of the face surface. The contour shape is a projected image obtained by projecting the contour line of the face surface to a plane. The plane to be projected is a plane perpendicular to a line connecting a center of gravity of the head and a sweet spot. The sweet spot is an intersection point of the face surface and a perpendicular line drawn from the center of gravity of the head to the face surface. The perpendicular line is a normal line of the face surface. 
     [FW Category] 
     “FW category” is defined as an original term of the present application. A club belonging to FW category satisfies the following specifications (1a) to (1e). 
     (1a) The head has a curved face surface. 
     (1b) The head has a hollow part. 
     (1c) The head has a volume of equal to or greater than 130 cc but equal to or less than 300 cc. 
     (1d) The head has a real loft of equal to or greater than 14 degrees but equal to or less than 33 degrees. 
     (1e) The club has a length of equal to or greater than 39.0 inches but equal to or less than 43.5 inches. 
     The specifications for FW category are typical specifications for a so-called fairway wood. 
     [HB Category] 
     “HB category” is defined as an original term of the present application. A club belonging to HB category satisfies the following specifications (2a) to (2e). 
     (2a) The head has a curved face surface. 
     (2b) The head has a hollow part. 
     (2c) The head has a volume of equal to or greater than 90 cc but less than 130 cc. 
     (2d) The head has a real loft of equal to or greater than 15 degrees but equal to or less than 33 degrees. 
     (2e) The club has a length of equal to or greater than 37.0 inches but equal to or less than 41.5 inches. 
     The specifications for HB category are typical specifications for a so-called hybrid type club. 
     [Club Length] 
     The club length is measured based on “1c Length” in “1 Clubs” of “Appendix II Design of Clubs” in the Golf Rules defined by R&amp;A (Royal and Ancient Golf club of Saint Andrews). The club length is measured in a state where a club is placed on a horizontal plane and a sole is set against a plane of which an angle with respect to the horizontal plane is 60 degrees. The method for measuring the club length is referred to as a 60-degrees method. 
     The crown c 1  includes a recess part RE 1 . The recess part RE 1  forms a groove. As shown in  FIG. 2 , the recess part RE 1  includes a toe-heel extension part RE 11  extending in the toe-heel direction, a slope part RE 12  slopingly extending to be positioned on a further back side as going to the toe side, and a slope part RE 13  slopingly extending to be positioned on a further back side as going to the heel side. The slope part RE 12  is connected to the toe side of the toe-heel extension part RE 11 . The slope part RE 13  is connected to the heel side of the toe-heel extension part RE 11 . 
     The slope part RE 12  has a slope angle θ 12  exceeding the permissible range)(±20°) of the toe-heel direction. In  FIG. 2  as an example, θ 12  is 45°. The slope part RE 13  has a slope angle θ 13  exceeding the permissible range (±20°) of the toe-heel direction. In  FIG. 2  as an example, θ 13  is 45°. The angle θ 12  and the angle θ 13  are, for example, preferably equal to or greater than 20° and more preferably equal to or greater than 25°. The angle θ 12  and the angle θ 13  are, for example, preferably equal to or less than 65° and more preferably equal to or less than 60°. The angles θ 12  and θ 13  are angles with respect to the toe-heel reference direction. These angles are measured on the planar view. 
     The recess part RE 1  divides the crown c 1 . The crown c 1  includes a back part  100  positioned at the back of the recess part RE 1 , and a front part  102  positioned at the front of the recess part RE 1 . 
       FIG. 6  shows a cross-sectional view taken along line F 2 -F 2  in  FIG. 2 . As mentioned above, the inside of the head h 1  is hollow.  FIG. 6  shows a cross section of only the crown c 1 . 
     As shown in  FIG. 6 , the recess part RE 1  includes a first side surface  104  positioned on the face side, and a second side surface  106  positioned on the back side. The recess part RE 1  further includes a bottom surface  108 . The bottom surface  108  may not be present. 
     A virtual extension line HL 1  is shown by a two-dot chain line in  FIG. 6 . The virtual extension line HL 1  is an extension line of the back part  100 . The virtual extension line HL 1  is determined at each cross section taken along the front-back direction. 
     The virtual extension line HL 1  is defined as follows. In a contour line of the surface of crown on a cross section taken along the front-back direction, a vertex of an angle on the back side of the recess part RE 1  is defined as point Pa, a point separated by 0.5 mm backward from the point Pa is defined as point P 1 , a point separated by 0.5 mm backward from the point P 1  is defined as point P 2 , and a point separated by 0.5 mm backward from the point P 2  is defined as point P 3  (See  FIG. 6 ). The virtual extension line HL 1  is a circle passing through the point P 1 , the point P 2  and the point P 3 . When the point P 1 , the point P 2  and the point P 3  are on a straight line, the virtual extension line HL 1  is a straight line passing through the point P 1 , the point P 2  and the point P 3 . The above mentioned “0.5 mm” is measured along the front-back reference direction. 
     When the recess part RE 1  is not present, the point Pa is set to a front end of the back part  100 . In this case, the point Pa is on a lower end of a step surface ST 1 . When the recess part RE 1  and the step surface ST 1  are separated, the point Pa is on the lower end of the step surface ST 1 . 
     When the point Pa is unclear due to roundness, the point Pa is set to a middle point in a portion having the smallest curvature radius. 
     In the present application, a virtual extension surface HF 1  is defined based on the virtual extension line HL 1 . The virtual extension surface HF 1  is a surface formed by a set of the virtual extension lines HL 1 . 
     As shown in  FIG. 6 , the crown c 1  includes the step surface ST 1 . The step surface ST 1  is positioned at a front of the back part  100 . The step surface ST 1  is positioned above the virtual extension line HL 1  (virtual extension surface HF 1 ). 
     As shown in  FIG. 6 , the step surface ST 1  is continuous with the first side surface  104 . The step surface ST 1  and the first side surface  104  form a continuous surface SR 1 . A boundary between the first side surface  104  and the step surface ST 1  is the virtual extension surface HF 1 . 
     The first side surface  104  forms the continuous surface SR 1  at all positions in the toe-heel direction. The step surface ST 1  is provided along the whole recess part RE 1 . The continuous surface SR 1  is provided along the whole recess part RE 1 . 
     A surface (outer surface) of the front part  102  connects an upper end of the step surface ST 1  (continuous surface SR 1 ) and an upper end of the face f 1 . The surface of the front part  102  forms a smooth curved surface extending between the upper end of the step surface ST 1  and the upper end of the face surface f 1 . 
     A surface (outer surface) of the back part  100  connects an upper end of the second side surface  106  and a back end of the crown c 1 . The surface of the back part  100  forms a smooth curved surface extending between the upper end of the second side surface  106  and the back end of the crown c 1 . 
     In the present embodiment, the step surface ST 1  extends along the recess part RE 1 . As a result, the continuous surface SR 1  extends along the whole recess part RE 1 . As shown in  FIG. 2 , a part (middle part) of the recess part RE 1  extends in the toe-heel direction, and a part of the step surface ST 1  also extends in the toe-heel direction. 
     The recess part RE 1  may be separated from the step surface ST 1 , although it is different from the present embodiment. The step surface ST 1  may be provided at a front of the recess part RE 1 . 
     In the head h 1 , deformation of the crown c 1  in hitting is promoted by the recess part RE 1  (effect of promoting deformation). The deformation increases a loft angle. Thus, a launch angle is increased, and backspin is increased. In addition, rebound performance is improved because of the promotion of deformation. 
     Hereinafter, hitting at a hitting point of an upper side of the face f 1  is also referred to as an “upper-side hitting”. In the upper-side hitting, backspin is likely to be decreased due to a longitudinal gear effect. In this case, it becomes difficult to stop the ball near a target (pin). In a shot of aiming at a target, an increased backspin is desired. In the upper-side hitting, a great force acts on the crown c 1 . Therefore, the above mentioned effect of promoting deformation is particularly effective in the upper-side hitting. The recess part RE 1  effectively restrains backspin from decreasing in the upper-side hitting. 
     The step surface ST 1  is a surface opened backward. There is no backup at the back of the step surface ST 1 . Therefore, in hitting, the step surface ST 1  can be deformed so as to fall backward. This deformation (falling deformation) can enhance the effect of promoting deformation (step-surface effect). 
     The virtual extension surface HF 1  may intersect the surface of the face f 1 . When hitting is performed at a point above the intersection line, the falling deformation is likely to occur. Therefore, the step-surface effect is further enhanced. 
     The back part  100  is disposed on a lower side than the front part  102  because of the presence of the step surface ST 1 . The low back part  100  can lower a center of gravity of the head. Although the presence of the recess part RE 1  can cause a disadvantage of making the center of gravity of the head higher, the low back part  100  can offset the disadvantage (offset effect). The low center of gravity contributes to a high launch angle, and facilitates a shot of aiming at a target. 
     The recess part RE 1  is visually recognized by the golf player at address. The recess part RE 1  extending in the toe-heel direction is almost parallel to the face surface. The recess part RE 1  can facilitate aiming the face surface toward a target. In other words, the recess part RE 1  can improve an alignment characteristic (alignment effect). The step surface ST 1  extending along the recess part RE 1  can further enhance the alignment characteristic. 
     In the embodiment of  FIG. 6 , the continuous surface SR 1  is formed. The continuous surface SR 1  is taller than the step surface ST 1 , and thereby being likely to be deformed in hitting. Not only the step surface ST 1  but also the whole continuous surface SR 1  can be deformed to fall backward (effect of increasing the falling deformation). For this reason, the step-surface effect is further enhanced. The continuous surface SR 1  enhances a synergistic effect of the recess-part effect and the step-surface effect. 
     A heel-divisional plane PL 1  is shown by a two-dot chain line in  FIG. 3 . The plane PL 1  is parallel to the axial line of the shaft. The plane PL 1  is brought into contact with an outer peripheral surface of the hosel z 1 . In the reference state, an intersection line of the plane PL 1  and the horizontal plane H is parallel to the front-back reference direction. 
     Of the crown c 1 , a portion at a back of the hosel z 1  is less likely to be deformed because of the presence of the hosel z 1 . In the head h 1 , the heel end of the recess part RE 1  is positioned on the heel side with respect to the heel-divisional plane PL 1 . The heel end of the step surface ST 1  is positioned on the heel side with respect to the heel-divisional plane PL 1 . Therefore, the heel side of the crown c 1  is likely to be deformed despite the presence of the hosel z 1 . 
     As shown in  FIG. 2 , the recess part RE 1  cuts across the crown c 1 . As shown in  FIG. 4 , an end Et on the toe side of the recess part RE 1  divides a contour line Lc of the crown c 1 . The end Et is positioned on the side part d 1 . As shown in  FIG. 5 , an end Eh on the heel side of the recess part RE 1  divides the contour line Lc of the crown c 1 . The end Eh is positioned on the side part d 1 . 
     As shown in  FIG. 2 , the recess part RE 1  continuously extends from the first end Et to the second end Eh thereof. The first end Et divides the contour line Lc at a first position, and the second end Eh divides the contour line Lc at a second position. The recess part RE 1  divides the surface of the crown c 1 . In the crown c 1 , the recess-part effect can spread to the whole face f 1 . For this reason, deformation of the crown c 1  can be further facilitated. The recess-part effect is enhanced by the recess part RE 1 . 
     The recess part RE 1  has a length longer than a face length. Therefore, the recess-part effect is enhanced. The length of the recess part RE 1  can be considered as a length of a line formed by a set of the points Pa. This length is a three-dimensional length. The face length is a maximum width of the face surface in the toe-heel reference direction. 
     When an extending direction of the recess part RE 1  is unclear, the extending direction of the line formed by a set of the points Pa is regarded as the extending direction of the recess part RE 1 . When an extending direction of the step surface ST 1  is unclear, an extending direction of the upper end of the step surface ST 1  is regarded as the extending direction of the step surface ST 1 . 
     An end on the toe side of the step surface ST 1  is positioned on the side part d 1 . An end on the heel side of the step surface ST 1  is positioned on the side part d 1 . The step surface ST 1  cuts across the crown c 1 . The step-surface effect can be improved by the step surface ST 1 . 
     An end on the toe side of the continuous surface SR 1  is positioned on the side part d 1 . An end on the heel side of the continuous surface SR 1  is positioned on the side part d 1 . The continuous surface SR 1  cuts across the crown c 1 . The synergistic effect of the recess part RE 1  and the step surface ST 1  is further enhanced by the continuous surface SR 1 . 
     A distance between the upper end of the face f 1  and the step surface ST 1  is shown by a double-pointed arrow T in  FIG. 2 . The distance T is measured along the front-back reference direction. The distance T is determined at each position in the toe-heel reference direction. 
     The shorter the distance T is, the nearer a distance between the step surface ST 1  and the hitting face is. It is considered that stress acting on a position becomes greater, as the position approaches the hitting face. Therefore, it is considered that as the distance T becomes shorter, the effect of promoting deformation becomes greater. The inventors of the present application, however, have found that there is an optimum value for the distance T. As shown in Examples below (Table 6), when T is equal to or greater than 5 mm, the effect of promoting deformation is great. The distance T is preferably equal to or greater than 5 mm, more preferably equal to or greater than 7 mm, and still more preferably equal to or greater than 9 mm. In light of the effect of promoting deformation, the distance T is equal to or less than 25 mm, and more preferably equal to or less than 20 mm. 
     As shown in  FIG. 6 , the first side surface  104  is inclined to be forward as going upward. The second side surface  106  is inclined to be backward as going upward. The step surface ST 1  is inclined to be forward as going upward. The inclination direction of the first side surface  104  is the same as the inclination direction of the step surface ST 1 . An interval between the first side surface  104  and the second side surface  106  becomes wider as going to the upper side. Therefore, a draft angle is secured. For this reason, the crown c 1  is easily formed. 
       FIG. 7  shows a perspective view of a head h 2 .  FIG. 8  shows a plan view of the head h 2 .  FIG. 8  is a figure viewed from the crown side.  FIG. 9  shows a side view of the toe side of the head h 2 .  FIG. 10  shows a side view of the heel side of the head h 2 .  FIG. 11( a )  shows a cross-sectional view taken along line F 11 -F 11  in  FIG. 8 . 
     The head h 2  is a wood type head. The head h 2  is a so-called fairway wood type. The inside of the head h 2  is hollow. In other words, the head h 2  has a hollow structure. 
     The head h 2  includes a crown c 2 , a sole s 2 , a hosel z 2  and a face f 2 . The crown c 2  extends toward the back side from an upper edge of the face f 2 . The sole s 2  extends toward the back side from a lower edge of the face f 2 . An outer surface of the face f 2  is a hitting face. The hitting face is also referred to as a face surface. 
     The head h 2  further includes a side part d 2 . The side part d 2  extends between the crown c 2  and the sole s 2 . The side part d 2  is also referred to as a skirt. 
     The crown c 2  includes a recess part RE 1 . The recess part RE 1  forms a groove. As shown in  FIG. 8 , the recess part RE 1  includes a toe-heel extension part RE 11  extending in the toe-heel direction, and a slope part RE 13  slopingly extending to be positioned on a further back side as going to the heel side. The slope part RE 13  is connected to the heel side of the toe-heel extension part RE 11 . 
     The recess part RE 1  divides the crown c 1 . The crown c 1  includes a back part  110  positioned at a back of the recess part RE 1 , and a front part  112  positioned at a front of the recess part RE 1 . 
       FIG. 11( a )  shows a cross-sectional view taken along line F 11 -F 11  in  FIG. 8 . The inside of the head h 2  is hollow, and  FIG. 11( a )  shows a cross section of only the crown c 2 . 
     As shown in  FIG. 11( a ) , the recess part RE 1  includes a first side surface  114  positioned on the face side and a second side surface  116  positioned on the back side. The recess part RE 1  further includes a bottom surface  118 . 
     As shown in  FIG. 11( a ) , the crown c 2  includes a step surface ST 1 . The step surface ST 1  is positioned at a front of the back part  110 . The step surface ST 1  is positioned above a virtual extension line HL 1  (virtual extension surface HF 1 ). 
     As shown in  FIG. 11( a ) , the step surface ST 1  is continuous with the first side surface  114 . The step surface ST 1  and the first side surface  114  form a continuous surface SR 1 . A boundary between the first side surface  114  and the step surface ST 1  is the virtual extension surface HF 1 . 
     A surface (outer surface) of the front part  112  connects an upper end of the step surface ST 1  (continuous surface SR 1 ) and an upper end of the face f 2 . The surface of the front part  112  forms a smooth curved surface extending between the upper end of the step surface ST 1  and the upper end of the face f 2 . 
     A surface (outer surface) of the back part  110  connects an upper end of the second side surface  116  and a back end of the crown c 2 . The surface of the back part  110  forms a smooth curved surface extending between the upper end of the second side surface  116  and the back end of the crown c 2 . 
     In the present embodiment, the continuous surface SR 1  is formed. The step surface ST 1  extends along the recess part RE 1 . As shown in  FIG. 8 , a part (other than the heel portion) of the recess part RE 1  extends in the toe-heel direction, and a part of the step surface ST 1  also extends in the toe-heel direction. 
     Also in the head h 2 , the recess-part effect, the step-surface effect, and the synergistic effect are exhibited. 
     Also in the embodiment of  FIG. 11( a ) , the continuous surface SR 1  is formed. As mentioned above, not only the step surface ST 1  but also the whole continuous surface SR 1  can be deformed to fall backward. For this reason, the step-surface effect is further enhanced. The continuous surface SR 1  enhances the synergistic effect of the recess-part effect and the step-surface effect. 
     As shown in  FIG. 8 , the recess part RE 1  cuts across the crown c 2 . As shown in  FIG. 9 , an end Et on the toe side of the recess part RE 1  divides a contour line Lc of the crown c 2 . The end Et is positioned on the side part d 2 . As shown in  FIG. 10 , an end Eh on the heel side of the recess part RE 1  divides the contour line to of the crown c 1 . The end Eh is positioned on the side part d 2 . 
     The recess part RE 1  having such a structure further facilitates deformation of the crown c 2 . The recess part RE 1  enhances the recess-part effect. 
     As mentioned above, the recess part RE 1  and the step surface ST 1  is unitary. An end on the toe side of the step surface ST 1  is positioned on the side part d 2 . An end on the heel side of the step surface ST 1  is positioned on the side part d 2 . The step surface ST 1  cuts across the crown c 2 . The step-surface effect can be improved by the step surface ST 1 . 
     An end on the toe side of the continuous surface SR 1  is positioned on the side part d 2 . An end on the heel side of the continuous surface SR 1  is positioned on the side part d 2 . The continuous surface SR 1  cuts across the crown c 2 . The synergistic effect is further enhanced by the continuous surface SR 1 . 
     As shown in  FIG. 8 , the head h 2  includes a rib rb 1 . The rib rb 1  is provided on an inner surface of the crown c 2 . The number of the ribs rb 1  may be one or plural. In the head h 2 , a plurality of (two) ribs rb 1  are provided. 
     The rib rb 1  is connected to the recess part RE 1 . The rib rb 1  intersects the recess part RE 1  (See  FIG. 8  and FIG.  11 ( a )). A front end of the rib rb 1  is positioned at a front of the recess part RE 1 . A back end of the rib rb 1  is positioned at a back of the recess part RE 1 . 
       FIG. 11( b )  shows a modified embodiment of a rib rb 2 . The rib rb 2  is connected to the recess part RE 1 . The rib rb 2  extends backward from a middle position in the width direction of the recess part RE 1 . 
     As mentioned above, the ribs rb 1  and rb 2  are connected to the recess part RE 1 . The ribs rb 1  and rb 2  can suppress the effect of promoting deformation because of the recess part RE 1 . The effect of promoting deformation can be controlled by disposal and rigidity of the rib. For example, the rib may be provided on only a middle region Rc in the toe-heel direction. This structure is effective in a case, for example, where a coefficient of restitution (COR) at the face center fc is excessively great. The rib can locally suppress deformation of the crown c 2 . 
       FIG. 12  shows a plan view of a head h 3 .  FIG. 13  is a cross-sectional view taken along line F 13 -F 13  in  FIG. 12 . 
     The head h 3  is a wood type head. The head h 3  is a so-called hybrid type. The inside of the head h 3  is hollow. In other words, the head h 3  has a hollow structure. 
     The head h 3  includes a crown c 3 , a sole (not shown in the drawings), a hosel z 3  and a face f 3 . The crown c 3  extends toward the back side from an upper edge of the face f 3 . The sole extends toward the back side from a lower edge of the face f 3 . An outer surface of the face f 3  is a hitting face. The head h 3  further includes a side part (not shown in the drawings). The side part extends between the crown c 3  and the sole. 
     The crown c 3  includes a recess part RE 1 . The recess part RE 1  forms a groove. As shown in  FIG. 12 , the recess part RE 1  includes a toe-heel extension part RE 11  extending in the toe-heel direction, a slope part RE 12  slopingly extending to be positioned on a further back side as going to the toe side, and a slope part RE 13  slopingly extending to be positioned on a further back side as going to the heel side. The slope part RE 12  is connected to the toe side of the toe-heel extension part RE 11 . The slope part RE 13  is connected to the heel side of the toe-heel extension part RE 11 . 
     The recess part RE 1  divides the crown c 3 . The crown c 3  includes a back part  120  positioned at a back of the recess part RE 1 , and a front part  122  positioned at a front of the recess part RE 1 . 
       FIG. 13  is a cross-sectional view taken along line F 13 -F 13  in  FIG. 12 . The inside of the head h 3  is hollow, and  FIG. 13  shows a cross section of only the crown c 3 . 
     As shown in  FIG. 13 , the recess part RE 1  includes a first side surface  124  positioned on the face side, and a second side surface  126  positioned on the back side. The recess part RE 1  further includes a bottom surface  128 . 
     As shown in  FIG. 13 , the crown c 3  includes a step surface ST 1 . The step surface ST 1  is positioned at a front of the back part  120 . The step surface ST 1  is positioned above the virtual extension line HL 1  (virtual extension surface HF 1 ). 
     As shown in  FIG. 13 , the step surface ST 1  is continuous with the first side surface  124 . The step surface ST 1  and the first side surface  124  form the continuous surface SR 1 . A boundary between the first side surface  124  and the step surface ST 1  is the virtual extension surface HF 1 . 
     Also in the head h 3 , the recess-part effect, the step-surface effect, and the offset effect are exhibited. In addition, the synergistic effect of the recess-part effect and the step-surface effect is enhanced because of the continuous surface SR 1 . 
       FIG. 14  shows a cross-sectional view of a crown c 4  of a head h 4  according to a modified embodiment. The crown c 4  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  includes a first side surface  134 , a second side surface  136 , and a bottom surface  138 . The step surface ST 1  is continuous with the first side surface  134  so as to form a continuous surface SR 1 . 
     As shown in  FIG. 14 , the first side surface  134  is inclined to be further backward as going upward. The second side surface  136  is inclined to be further backward as going upward. The step surface ST 1  is inclined to be further backward as going upward. An inclination direction of the first side surface  134  is the same as an inclination direction of the step surface ST 1 . The inclination direction of the first side surface  134  is the same as an inclination direction of the second side surface  136 . An interval between the first side surface  134  and the second side surface  136  is constant regardless of the position in the up-down direction. Therefore, it is possible to extract a mold. 
     An apparent width of the recess part RE 1  is shown by a double-pointed arrow V 1  in  FIG. 14 . The width V 1  shows a width of the recess part RE 1  which is visually recognized at address. Since the first side surface  134  is inclined backward, a part of the recess part RE 1  is hidden by the first side surface  134 . In addition, since the step surface ST 1  is inclined backward, a part of the recess part RE 1  is hidden by the step surface ST 1 . As a result, the apparent width V 1  is made small. The apparent width V 1  is suppressed while the volume of the recess part RE 1  is secured. Because of the small apparent width V 1 , the recess part RE 1  becomes inconspicuous. 
     As mentioned above, the recess part RE 1  can produce the alignment effect. However, some golf players can have an uncomfortable feeling because of the visual recognition of the recess part RE 1 . The uncomfortable feeling can also be caused by the extending direction of the recess part RE 1  and the like. The uncomfortable feeling due to the recess part RE 1  can be suppressed by making the recess part RE 1  inconspicuous. 
     Since the first side surface  134  is inclined backward, the first side surface  134  is likely to fall backward. Therefore, the falling deformation can be easily produced. In addition, since the continuous surface SR 1  including the step surface ST 1  is inclined backward, the effect of increasing the falling deformation is enhanced. Because of these facts, deformation of the crown c 4  is further promoted. 
       FIG. 15  shows a cross-sectional view of a crown c 5  of a head h 5  according to another modified embodiment. The crown c 5  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  includes a first side surface  144 , a second side surface  146 , and a bottom surface  148 . The step surface ST 1  is continuous with the first side surface  144  so as to form a continuous surface SR 1 . 
     The recess part RE 1  includes a first side part  150 , a second side part  152  and a bottom part  154 . The first side part  150  is a portion having the first side surface  144  as a surface thereof. The second side part  152  is a portion having the second side surface  146  as a surface thereof. The bottom part  154  is a portion having the bottom surface  148  as a surface thereof. The step part  156  is a portion having the step surface ST 1  as a surface thereof. 
     The bottom part  154  has a thickness greater than a thickness of the first side part  150 . The thickness of the bottom part  154  is greater than a thickness of the second side part  152 . The thickness of the bottom part  154  is greater than a thickness of the step part  156 . By thickening only the bottom part  154 , durability can be improved while reduction of the effect of promoting deformation is suppressed. Thus, the bottom part  154  preferably has the maximum thickness in the recess part RE 1 . 
     It is preferable that a thickness of at least a part of the recess part RE 1  is greater than a minimum thickness of the crown. In this case, durability of the recess part RE 1  can be improved while a weight of the crown is suppressed. 
       FIG. 16  shows a cross-sectional view of a crown c 6  of a head h 6  according to another modified embodiment. The crown c 6  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  includes a first side surface  156 , a second side surface  158 , and a bottom surface  160 . The step surface ST 1  is continuous with the first side surface  156  so as to form a continuous surface SR 1 . 
     The crown c 6  includes a filler  162 . The filler  162  is disposed inside the recess part RE 1 . The filler  162  occupies at least a part of the recess part RE 1 . In the present embodiment, the filler  162  occupies the whole recess part RE 1 . The filler  162  covers the whole first side surface  156 . The filler  162  covers the whole second side surface  158 . The filler  162  covers the whole bottom surface  160 . An upper surface of the filler  162  is substantially equivalent to the virtual extension surface HF 1 . The substantially equivalent means that a difference in the up-down direction is equal to or less than 0.2 mm. 
     The filler  162  make the recess part RE 1  inconspicuous. Therefore, the uncomfortable feeling at address due to the recess part RE 1  can be suppressed. 
     By appropriately selecting a material for the filler  162 , the filler  162  does not hamper deformation of the recess part RE 1 . In addition, the filler  162  can produce a vibration absorbing effect. The vibration absorbing effect can enhance durability of the recess part RE 1 . 
     In light of not hampering deformation of the crown, and in light of vibration absorption, the material of the filler  162  is preferably a polymer. Examples of the polymer include an elastomer (including a rubber) and a resin. 
     More specifically, examples of the polymer include a thermosetting polymer and a thermoplastic polymer. Examples of the thermosetting polymer include a phenol resin, an epoxy resin, a melamine resin, a urea resin, an unsaturated polyester resin, an alkyd resin, a thermosetting polyurethane, a thermosetting polyimide, and a thermosetting elastomer. Examples of the thermoplastic polymer include polyethylene, polypropylene, polyvinyl chloride, polystyrene, polytetrafluoroethylene, an ABS resin (acrylonitrile butadiene styrene resin), an acrylic resin, polyamide, polyacetal, polycarbonate, modified polyphenylene ether, polybutylene terephthalate, polyethylene terephthalate, polyphenylene sulfide, polyether ether ketone, a thermoplastic polyimide, polyamide imide, and a thermoplastic elastomer. 
     Examples of the thermoplastic elastomer include a thermoplastic polyamide elastomer, a thermoplastic polyester elastomer, a thermoplastic polystyrene elastomer, a thermoplastic polyester elastomer, and a thermoplastic polyurethane elastomer. 
     In light of durability, a urethane-based polymer and polyamide are preferable, and the urethane-based polymer is more preferable. Examples of the urethane-based polymer include polyurethane and a thermoplastic polyurethane elastomer. The urethane-based polymer may be thermoplastic, or may be thermosetting. 
     In light of formability, a thermoplastic polymer is preferable. In light of a hardness and durability, in the thermoplastic polymer, the polyamide and the thermoplastic polyurethane elastomer are preferable, and the thermoplastic polyurethane elastomer is more preferable. 
     Examples of the polyamide include nylon 6, nylon 11, nylon 12, and nylon 66. 
     A preferable thermoplastic polyurethane elastomer contains a polyurethane component as a hard segment, and a polyester component or a polyether component as a soft segment. That is, preferable examples of the thermoplastic polyurethane elastomer (TPU) include a polyester-based TPU and a polyether-based TPU. Examples of a curing agent for the polyurethane component include cycloaliphatic diisocyanate, aromatic diisocyanate, and aliphatic diisocyanate. 
     Commercially available examples of the thermoplastic polyurethane elastomer (TPU) include trade name “Elastollan” manufactured by BASF Japan Ltd. 
       FIG. 17  shows a cross-sectional view of a crown c 7  of a head h 7  according to another modified embodiment. The crown c 7  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  includes a first side surface  164 , a second side surface  166  and a bottom surface  168 . The step surface ST 1  is continuous with the first side surface  164  so as to form a continuous surface SR 1 . 
     The crown c 7  includes a lid member  170 . The lid member  170  covers an opening of the recess part RE 1 . The lid member  170  make the recess part RE 1  inconspicuous. Therefore, the uncomfortable feeling at address due to the recess part RE 1  can be suppressed. 
       FIG. 18( a )  to  FIG. 24( c )  show heads according to modified embodiments. 
     In a head h 8  of  FIG. 18( a ) , a crown c 8  includes a step surface ST 1  and a recess part RE 1 . The step surface ST 1  is an inclined surface that is inclined with respect to the up-down direction. The step surface ST 1  is clearly visible in the planar view (See  FIG. 18( a ) ). The step surface ST 1  includes a first portion  180 , a second portion  182  connected to the toe end of the first portion  180  and extends backward, and a third portion  184  connected to the heel end of the first portion  180  and extends backward. The recess part RE 1  is disposed on a back of the first portion  180 . The recess part RE 1  extends in the toe-heel direction. 
     An angle between the step surface ST 1  and the up-down direction is preferably equal to or less than 60°, more preferably equal to or less than 45°, and still more preferably equal to or less than 30°. 
     In a head h 9  of  FIG. 18( b ) , a crown c 9  includes a step surface ST 1  and a recess part RE 1 . The recess part RE 1  and the step surface ST 1  form a continuous surface SR 1 . The continuous surface SR 1  and the recess part RE 1  include a first portion  186  that extends in the toe-heel direction, a second portion  188  connected to the heel end of the first portion  186 , and a third portion  190  connected to the back end of the second portion  188 . The toe end of the first portion  186  extends to reach a side part. The second portion  188  slopingly extends to be positioned on a further heel side as going backward. The third portion  190  extends toward the toe side from the back end of the second portion  188 . The toe end of the third portion  190  extends to reach the side part. 
     In a head h 10  of  FIG. 18( c ) , a crown c 10  includes a step surface ST 1  and a recess part RE 1 . The recess part RE 1  and the step surface ST 1  form a continuous surface SR 1 . The continuous surface SR 1  and the recess part RE 1  include a first portion  192  that extends in the toe-heel direction, a second portion  194  connected to the toe end of the first portion  192 , and a third portion  196  connected to the heel end of the first portion  192 . The toe end of the second portion  194  extends to reach a side part. The second portion  194  slopingly extends to be positioned on a further toe side as going backward. The heel end of the third portion  196  extends to reach the side part. The third portion  196  slopingly extends to be positioned on a further heel side as going backward. The heel end of the third portion  196  is positioned on a back with respect to the toe end of the second portion  194 . 
     In a head h 11  of  FIG. 19( a ) , a crown c 11  includes a step surface ST 1  and a recess part RE 1 . The crown c 11  includes the above described continuous surface SR 1 . The continuous surface SR 1  and the recess part RE 1  include a first portion  198  that extends in the toe-heel direction, a second portion  200  connected to the toe end of the first portion  198 , and a third portion  202  connected to the heel end of the first portion  198 . The second portion  200  includes a portion extending in the front-back direction. The back end of the second portion  200  extends to reach a side part. The heel end of the third portion  202  extends to reach the side part. The third portion  202  slopingly extends to be positioned on a further heel side as going backward. The back end of the second portion  200  is positioned on a back with respect to the back end of the third portion  202 . 
     In a head h 12  of  FIG. 19( b ) , a crown c 12  includes a step surface ST 1  and a recess part RE 1 . The recess part RE 1  and the step surface ST 1  form a continuous surface SR 1 . The continuous surface SR 1  is disconnected at a middle region Rc in the toe-heel direction. The recess part RE 1  is disconnected at the middle region Rc in the toe-heel direction. The step surface ST 1  is disconnected at the middle region Rc in the toe-heel direction. 
     The middle region Rc in the toe-heel direction is described as follows. When a position separated by 10 mm toward the toe side from the face center fc is defined as Pt, and a position separated by 10 mm toward the heel side from the face center fc is defined as Ph, the middle region in the toe-heel direction means a region between the position Pt and the position Ph. These “10 mm” is measured along the toe-heel reference direction. 
     The recess part RE 1  and the step surface ST 1  include a first portion  204  that extends toward the toe side to reach a side part from the middle region in the toe-heel direction, and a second portion  206  that extends toward the heel side to reach the side part from the middle region in the toe-heel direction. The recess part RE 1  is not formed between the first portion  204  and the second portion  206 , and neither is the step surface ST 1 . The middle region Rc in the toe-heel direction of the crown c 12  includes a region in the toe-heel reference direction in which neither the recess part RE 1  nor the step surface ST 1  is present. This structure is effective in a case where, for example, the coefficient of restitution (COR) near the face center fc is excessively large. This structure can enhance the COR around the face while suppressing the COR near the face center fc. Therefore, equalization in distribution of COR is achieved. 
     In a head h 13  of  FIG. 19( c ) , a crown c 13  includes a step surface ST 1  extending in the toe-heel direction, and a recess part RE 1  extending along the step surface ST 1 . The recess part RE 1  is shorter than the step surface ST 1 . The recess part RE 1  is provided along a middle part of the step surface ST 1 . The recess part RE 1  is provided on the middle region Rc in the toe-heel direction. In this structure, the COR near the face center fc can be particularly enhanced. This structure can selectively enhance deformation of the crown c 13  near the face center fc. 
     In a head h 14  of  FIG. 20( a ) , a crown c 14  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  and the step surface ST 1  form a continuous surface SR 1 . The recess part RE 1  and the step surface ST 1  does not cut across the crown c 14 . Toe ends of the recess part RE 1  and the step surface ST 1  do no reach a side part. Heel ends of the recess part RE 1  and the step surface ST 1  do not reach the side part. The recess part RE 1  is shorter than the face length. The step surface ST 1  is shorter than the face length. The recess part RE 1  and the step surface ST 1  can partially promote deformation of the crown c 14 . 
     The recess part RE 1  and the step surface ST 1  are provided on the middle region Rc in the toe-heel direction. This structure can selectively enhance the COR in middle region Rc in the toe-heel directionally. This structure can selectively enhance deformation of the crown c 14  near the face center fc. 
     In the planer view, the step surface ST 1  and the recess part RE 1  are curved so as to protrude toward the face side. For this reason, the distance T (see  FIG. 2 ) is gradually varied. By varying the distance T, deformation of the crown c 14  is controlled at each position in the toe-heel reference direction. 
     In a head h 15  of  FIG. 20( b ) , a crown c 15  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  and the step surface ST 1  form a continuous surface SR 1 . The recess part RE 1  and the step surface ST 1  include a first portion  208 , a second portion  210  connected to the toe side of the first portion  208 , and a third portion  212  connected to the heel side of the first portion  208 . The second portion  210  slopingly extends to be positioned on a further toe side as going backward. The third portion  212  slopingly extends to be positioned on a further heel side as going backward. In comparison between the distances T (see  FIG. 2 ), the distance T of the first portion  208  is shorter than the distance T of the second portion  210 . The distance T of the first portion  208  is shorter than the distance T of the third portion  212 . In the second portion  210 , the distance T becomes longer as going to the toe side. In the third portion  212 , the distance T becomes longer as going to the heel side. Because of the distribution of the distance T, deformation of the crown c 15  is controlled at each position in the toe-heel reference direction. The toe-heel directional region of the first portion  208  includes the face center fc. In this structure, the crown c 15  is largely deformed at the face center fc. 
     In a head h 16  of  FIG. 20( c ) , a crown c 16  includes a recess part RE 1  and the step surface ST 1 . The recess part RE 1  and the step surface ST 1  form a continuous surface SR 1 . The recess part RE 1  and the step surface ST 1  include a first portion  216 , a second portion  218  connected to the toe side of the first portion  216 , and a third portion  220  connected to the heel side of the first portion  216 . In comparison between the distances T (see  FIG. 2 ), the distance T of the first portion  216  is shorter than the distance T of the second portion  218 . The distance T of the first portion  216  is shorter than the distance T of the third portion  220 . Because of the distribution of the distance T, deformation of the crown c 16  is controlled at each position in the toe-heel reference direction. The first portion  216  is positioned on the face center fc. In this structure, the crown c 16  is particularly largely deformed at the face center fc. 
     In a head h 17  of  FIG. 21( a ) , the crown c 17  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  and the step surface ST 1  form a continuous surface SR 1 . The recess part RE 1  and the step surface ST 1  include a first portion  222 , a second portion  224  connected to the toe side of the first portion  222 , and a third portion  226  connected to the heel side of the first portion  222 . The recess part RE 1  and the step surface ST 1  further include a fourth portion  228  connected to the toe side of the first portion  222 , and a fifth portion  230  connected to the heel side of the first portion  222 . The fourth portion  228  is provided at a back of the second portion  224 . The fifth portion  230  is provided at a back of the third portion  226 . 
     The fourth portion  228  slopingly extends to be positioned on a further toe side as going backward. The fifth portion  230  slopingly extends to be positioned on a further heel side as going backward. 
     The first portion  222  is provided on the middle region Rc in the toe-heel direction. The first portion  222  has a recess-part width greater than recess-part widths of other portions. The greater recess-part width promotes deformation of the crown c 17 . 
     In a head h 18  of  FIG. 21( b ) , a crown c 18  includes a recess part RE 1  and a step surface ST 1  (a first step surface). The crown c 18  further includes a second step surface  232 . The step surface  232  is provided on a back of the step surface ST 1 . The step surface  232  includes a first portion  234 , a second portion  236  connected to the toe end of the first portion  234 , and a third portion  238  connected to the heel end of the first portion  234 . 
     The first portion  234  is provided on the middle region Rc in the toe-heel direction. The first portion  234  is closer to the first step surface ST 1  than the second portion  236 . The first portion  234  is closer to the first step surface ST 1  than the third portion  238 . Therefore, in a region in which the first portion  234  is present, deformation of the crown c 18  is likely to be increased. By a synergistic effect of the two step surfaces, deformation of the crown c 18  can be controlled at each position in the toe-heel reference direction. 
     In a portion positioned at a back of the step surface  232 , the height of the crown c 18  is suppressed. The lower portion contributes to lowering the center of gravity of the head. 
     In a head h 19  of  FIG. 21( c ) , a crown c 19  includes a recess part RE 1  and a step surface ST 1 . The step surface ST 1  includes a first portion  240 , a second portion  242  connected to the toe side of the first portion  240 , and a third portion  244  connected to the heel side of the first portion  240 . The recess part RE 1  includes a toe portion  246  provided along the second portion  242 , and a heel portion  248  provided along the third portion  244 . A recess part RE 1  along the first portion  240  is not provided. 
     Thus, the step surface ST 1  includes portions  242  and  244  accompanied with the recess part RE 1 , and a portion  240  not accompanied with the recess part RE 1 . Because of absence of the recess part RE 1 , deformation of the crown c 19  is relatively suppressed in a region in which the first portion  240  is present. This structure can contribute to equalization of the COR. 
     In a head h 20  of  FIG. 22( a ) , a crown c 20  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  and the step surface ST 1  include a bent part  250  that is bent so as to be recessed backward. The bent part  250  is provided on the middle region Rc in the toe-heel direction. The bent part  250  includes a first portion  254  that extends toward the toe side from a vertex  252 , and a second portion  256  that extends toward the heel side from the vertex  252 . Because of the bent part  250 , the recess part RE 1  and the step surface ST 1  are long. The long recess part RE 1  and the long step surface ST 1  can promote deformation of the crown c 19 . 
     In a head h 21  of  FIG. 22( b ) , a crown c 21  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  and the step surface ST 1  include a first portion  258 , a second portion  260  connected to the toe end of the first portion  258 , and a third portion  262  connected to the heel end of the first portion  258 . The second portion  260  includes a portion that extends in the front-back direction. The back end of the second portion  260  reaches a side part. The third portion  262  includes a portion that extends in the front-back direction. The back end of the third portion  262  reaches the side part. In the head, since the recess part RE 1  has a wide width, deformation of the crown c 21  is promoted. 
     In a head h 22  of  FIG. 22( c ) , a crown c 22  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  includes a width-variation part  264  of which the recess-part width is increased as going to the toe side. The recess-part width is an opening width of the recess part, and is measured along the front-back reference direction. This structure can promote deformation in the toe side. 
     In a head h 23  of  FIG. 23( a ) , a crown c 23  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  and the step surface ST 1  include a first portion  266 , a second portion  268  connected to the toe side of the first portion  266 , and a third portion  270  connected to the heel side of the first portion  266 . The distance T (see  FIG. 2 ) of the first portion  266  is greater than the distance T of the second portion  268 . The distance T of the first portion  266  is greater than the distance T of the third portion  270 . The first portion  266  is positioned in middle region Rc in the toe-heel direction. Because of the greater distance T, deformation of the middle region Rc in the toe-heel direction is suppressed. 
     In a head h 24  of  FIG. 23( b ) , a crown c 24  includes a recess part RE 1  and a step surface ST 1 . Toe ends of the recess part RE 1  and the step surface ST 1  reach a side part. Heel ends of the recess part RE 1  and the step surface ST 1  reach the side part. In the planer view, the step surface ST 1  and the recess part RE 1 , as a whole, are curved so as to protrude toward the face side. For this reason, the distance T (see  FIG. 2 ) is gradually varied. By varying the distance T, deformation of the crown c 24  is controlled at each position in the toe-heel reference direction. 
     In a head h 25  of  FIG. 23( c ) , a crown c 25  includes a recess part RE 1  and a step surface ST 1 . Toe ends of the recess part RE 1  and the step surface ST 1  reach a side part. Heel ends of the recess part RE 1  and the step surface ST 1  reach the side part. The recess part RE 1  includes a first portion  272  and a second portion  274 . The first portion  272  is connected to the toe end of the second portion  274 . The recess-part width of the first portion  272  is greater than the recess-part width of the second portion  274 . In a region in which the first portion  272  is present, deformation of the crown c 25  is further promoted. 
     In a head h 26  of  FIG. 24( a ) , a crown c 26  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  and the step surface ST 1  include a first portion  276 , a second portion  278  connected to the toe side of the first portion  276 , and a third portion  280  connected to the heel side of the first portion  276 . The distance T (see  FIG. 2 ) of the first portion  276  is greater than the distance T of the second portion  278 . The distance T of the first portion  276  is greater than the distance T of the third portion  280 . The first portion  276  is positioned on the middle region Rc in the toe-heel direction. In the planer view, the first portion  276  is curved so as to protrude backward. Based on the variation of the distance T, deformation is controlled at each position in the toe-heel reference direction. 
     In a head h 27  of  FIG. 24( b ) , a crown c 27  includes a recess part RE 1  and a step surface ST 1 . The recess part RE 1  and the step surface ST 1  include a first portion  282 , a second portion  284  connected to the toe side of the first portion  282 , and a third portion  286  connected to the heel side of the first portion  282 . The recess-part width of the first portion  282  is greater than the recess-part width of the second portion  284 . The recess-part width of the first portion  282  is greater than the recess-part width of the third portion  286 . Because of the greater recess-part width, the crown c 27  is largely deformed in a region in which the first portion is present. 
     Toe ends of the recess part RE 1  and the step surface ST 1  reach a side part. Heel ends of the recess part RE 1  and the step surface ST 1  do not reach the side part. Therefore, the toe side is largely deformed as compared with the heel side. 
     In a head h 28  of  FIG. 24( c ) , a crown c 28  includes a recess part RE 1  and a step surface ST 1  (first step surface). The crown c 28  further includes a second step surface  288 , a third step surface  290 , and a fourth step surface  292 . The plurality of step surfaces stepwise lower the up-down directional position of the crown c 28 . The head h 28  has a low center of gravity of the head. 
       FIG. 25  shows a perspective view of a head h 29 .  FIG. 26  shows a plan view of the head h 29 .  FIG. 27  shows a cross-sectional view taken along line F 27 -F 27  in  FIG. 26 . 
     The head h 29  does not include a recess part RE 1 . The head h 29  includes a step surface ST 1 . As shown in  FIG. 27 , the step surface ST 1  is an inclined surface. The step surface ST 1  is inclined to be closer to the face side as going upward. As described later, in a set of the present invention, a head not having a recess part RE 1  can be used. 
       FIG. 28  shows a perspective view of a head h 30 .  FIG. 29  shows a plan view of the head h 30 .  FIG. 30  shows a front view of the head h 30 .  FIG. 31  shows a toe-side view of the head h 30 .  FIG. 32  is a cross-sectional view taken along line F 32 -F 32  in  FIG. 29 . 
     The head h 30  includes a crown c 30 , a sole s 30 , a face f 30 , and a hosel z 30 . 
     The crown c 30  includes a back part  300 , a step surface ST 1  positioned at a front of the back part  300  and positioned above the virtual extension surface of the back part  300 , and a recess part RE 1  extending in the front-back direction. The crown c 30  further includes a front part  304  positioned at a front of the step surface ST 1 . 
     When the extending direction of the recess part RE 1  is unclear, the extending direction of the width-directional center line of the recess part RE 1  can be regard as the extending direction of the recess part RE 1 . The width-directional center line is a set of central points in the toe-heel reference direction. 
     The definition of the virtual extension surface is as described above. In the present embodiment, the point Pa is a front end  302  of the back part  300 . The front end  302  is a boundary between the back part  300  and the step surface ST 1 . The boundary is determined on a cross section along the front-back reference direction. When the boundary is unclear, a central point of a portion having the smallest curvature radius in the cross section is set to the front end  302 . 
     A plurality of the recess parts RE 1  are provided. The recess parts RE 1  are provided at respective toe-heel reference directional positions. 
     The plurality of recess parts RE 1  include the recess part RE 1  positioned on the middle region Rc in the toe-heel direction (see  FIG. 19( b ) ). The plurality of recess parts RE 1  include the recess part RE 1  positioned on the toe side with respect to the middle region Rc in the toe-heel direction. The plurality of recess parts RE 1  include the recess part RE 1  positioned on the heel side with respect to the middle region Rc in the toe-heel direction. 
     The recess part RE 1  intersects the step surface ST 1 . In other words, a front end  306  of the recess part RE 1  is positioned at a front of the step surface ST 1  or positioned on the step surface ST 1 , and a back end  308  of the recess part RE 1  is positioned at a back of the step surface ST 1 . 
     The recess part RE 1  includes a depth-variation part  310  having a depth D of decreasing as going backward. The depth D of the recess part RE 1  extending in the front-back direction is measured on a cross section along the toe-heel reference direction. In the cross section (not shown in the drawings), the depth D is determined based on a straight line covering the opening of the recess part RE 1 . The depth-variation part  310  extends to reach the back end  308 . The depth D at the back end  308  is zero. The depth-variation part  310  can disperse stress that acts on the step surface ST 1  when the crown c 30  is deformed. Therefore, durability of the step surface ST 1  can be improved. 
     The recess part RE 1  that extends in the front-back direction promotes deformation of the crown c 30 . At impact, the crown c 30  is compressed in the front-back direction. In conjunction with the compressive deformation, the crown c 30  is deformed so that a top part thereof is upwardly swelled. The deformation is also referred to as a swelling deformation. The swelling deformation includes an elongated deformation in which the crown c 30  is elongated in the toe-heel direction. The recess part RE 1  extending in the front-back direction can function as a room of elongation for the elongated deformation. As a result, the recess part RE 1  extending in the front-back direction promotes the deformation of the crown c 30  at impact. Therefore, also in the crown c 30 , the effect of promoting deformation is exhibited. In addition, the step-surface effect is exhibited because of the step surface ST 1 . 
     The material of the head is not limited. Examples of the material of the head include a metal, CFRP (carbon fiber reinforced plastic), and the like. Examples of the metal include one or more kinds selected from soft iron, pure titanium, a titanium alloy, stainless steel, maraging steel, an aluminium alloy, a magnesium alloy, and a tungsten-nickel alloy. Examples of the stainless steel include SUS630 and SUS304. Examples of the titanium alloy include 6-4 titanium (Ti-6Al-4V), Ti-15V-3Cr-3Sn-3Al, Ti-6-22-22S, and the like. The soft iron means low carbon steel having a carbon content of less than 0.3 wt %. 
     The volume of the head is not limited. In a small head, the crown is less likely to be deformed. Therefore, the present invention is effective in a head having a small volume. In this respect, the volume of the head is preferably equal to or less than 470 cc, and more preferably equal to or less than 300 cc. In view of a sweet area, the volume of the head is preferably equal to or greater than 90 cc. 
     The height (SS height) of a sweet spot is no limited. In a head having a low center of gravity, dropping of a golf ball during flight is likely to occur in the upper-side hitting due to insufficient backspin. Therefore, the present invention is effective in a head having a low center of gravity. In this respect, when the SS height is defined as Hs (mm), and the height of the face center fc is defined as Hc (mm), then a difference (Hs−Hc) is preferably equal to or less than 8 mm, and more preferably equal to or less than 6 mm. The height Hs is measured along the up-down direction of the head in the reference state. The height Hc is measured along the up-down direction of the head in the reference state. The difference (Hs−Hc) is preferably equal to or greater than 4 mm. 
     As mentioned above, the present invention is effective in a head having a low center of gravity. In this respect, the SS height Hs is preferably equal to or less than 26 mm, and more preferably equal to or less than 25 mm. The SS height Hs is preferably equal to or greater than 24 mm. 
     The recess part RE 1  may be painted by the same color as a color of other parts of the crown. In this case, the recess part RE 1  becomes inconspicuous. The presence of the recess part RE 1  may cause a visually uncomfortable feeling at address. The painting can suppress the visually uncomfortable feeling. 
     The recess part RE 1  may be painted by the same color as a color of other parts of the crown, and only the recess part RE 1  may be subjected to matte painting. The matte painting suppresses a gloss. Therefore, the visually uncomfortable feeling can be further suppressed. 
     [Embodiments of Golf Club Sets] 
     The present application includes an invention of a golf club set. The above described heads can be used for the golf club set. The set includes at least one of the heads. 
       FIG. 33  shows a golf club  4 . The golf club  4  includes a head  6 , a shaft  8 , and a grip  10 . The head  6  is attached to a tip part of the shaft  8 . The grip  10  is attached to a butt part of the shaft  8 . 
       FIG. 34  shows a golf club set  2 . The set  2  includes a plurality of golf clubs  4 . The set  2  of the present embodiment includes four golf clubs  4 . Each golf club  4  includes a head  6 , a shaft  8 , and a grip  10 . 
     The number of clubs  4  in the set  2  is equal to or greater than 2. In view of the restriction on the number of clubs in the rule, the number of the clubs  4  in the set  2  is preferably equal to or less than 8, more preferably equal to or less than 7, and still more preferably equal to or less than 6. The set  2  may not include a driver (a number 1 wood). The set  2  may be constituted with clubs of only FW category. The set  2  may be constituted with clubs of only HB category. The set  2  may be constituted with a club of FW category and a club of HB category only. The set  2  may include a driver. In a set described later, the number of clubs is indicated by an integer N, an integer M or an integer Q. 
     In the set  2 , the type of the head  6  is not limited. The head  6 , for example, may be wood type or hybrid type. A wood type head and a hybrid type head may be used in combination. 
     In the present embodiment, the set  2  includes a club  41 , a club  42 , a club  43  and a club  44  in an order from the club having the smallest loft angle. The club  41  has a head  61 . The club  42  has a head  62 . The club  43  has a head  63 . The club  44  has a head  64 . The club  41  has a shaft  81 . The club  42  has a shaft  82 . The club  43  has a shaft  83 . The club  44  has a shaft  84 . 
     As shown in  FIG. 34 , the real loft R of the club  41  is shown by a double-pointed arrow La 1 . The real loft R of the club  42  is shown by a double-pointed arrow La 2 . The real loft R of the club  43  is shown by a double-pointed arrow La 3 . The real loft R of the club  44  is shown by a double-pointed arrow La 4 . Magnitude relationship between the real lofts R is La 1 &lt;La 2 &lt;La 3 &lt;La 4 . The set  2  satisfies the following relationship A.
         [Relationship A]: The shorter the club length is, the greater the real loft R is.       

     The set according to the present invention may not satisfy the relationship A. For example, it is common for a set including a club of HB category and a club of FW category in combination not to satisfy the relationship A. 
     At least one of the heads  61  to  64  which constitute the set  2  includes the recess part RE 1  and the step surface ST 1 . All of the heads  61  to  64  which constitute the set  2  may include the recess part RE 1  and the step surface ST 1 . 
     The set  2  may include a head that does not have the recess part RE 1  and that has the step surface ST 1 . An example for this head is the head h 29  ( FIG. 25 ). 
     In the set  2 , the depth D and the height H for each club number may be different from those of other club numbers. The difference is useful for optimizing the function for each club number. 
     Table 1 below shows specifications of a club set A of a first embodiment. The set A includes a club of FW category and a club of HB category. The set A includes a club having a depth D of zero. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Table 1 Specifications of set A 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Recess- 
                   
                   
               
               
                   
                   
                 Real 
                 Club 
                   
                 part  
                   
                   
               
               
                   
                   
                 loft  
                 length  
                 Height 
                 depth  
                   
                   
               
               
                   
                   
                 R 
                 L 
                 H 
                 D 
                 X 
                 Y 
               
               
                   
                   
                 (degree) 
                 (inch) 
                 (mm) 
                 (mm) 
                 (R/L) 
                 (D × H) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Set A1 
                 #3 
                 15 
                 43 
                 1.5 
                 0 
                 0.35 
                 0.00 
               
               
                 (FW 
                 #4 
                 17 
                 42.75 
                 1.5 
                 0 
                 0.40 
                 0.00 
               
               
                 category) 
                 #5 
                 19 
                 42.5 
                 1.5 
                 1 
                 0.45 
                 1.50 
               
               
                   
                 #7 
                 21 
                 42 
                 1.5 
                 2 
                 0.50 
                 3.00 
               
               
                 Set A2 
                 #2 
                 16 
                 40.75 
                 1.5 
                 1 
                 0.39 
                 1.50 
               
               
                 (HB 
                 #3 
                 19 
                 40.25 
                 1.5 
                 2 
                 0.47 
                 3.00 
               
               
                 category) 
                 #4 
                 22 
                 39.75 
                 1.5 
                 3 
                 0.55 
                 4.50 
               
               
                   
                 #5 
                 25 
                 39.25 
                 1.5 
                 3 
                 0.64 
                 4.50 
               
               
                   
               
            
           
         
       
     
     The set A includes a club set A1 of FW category. The set A1 includes a plurality of clubs belonging to FW category. Specifically, the set A1 includes four clubs belonging to FW category. Heads used for the set A1 have the same structure as the head h 1 . As described later, however, the set A1 includes clubs having a recess-part depth D of zero. 
     In the set A1, the greater the real loft R is, the shorter the club length L is. In the set A1, the heights H are constant. In the set A1, the depths D are varied. In the set A1, as the club length L becomes shorter, the depth D tends to be greater. 
     In the recess part RE 1  extending in the toe-heel direction, the recess-part depth D means a depth from the virtual extension line HL 1 . The recess-part depth D is measured along the up-down direction. 
     The height H is also measured along the up-down direction. The height H is a height from the virtual extension line HL 1 . 
     When the number of clubs of the set A1 is defined as N (N is an integer of equal to or greater than 2), and the recess-part depths D of the clubs are defined as D 1 , D 2 , . . . , Dn in an order from the club having the greatest length L, the set A1 satisfies the following relationship F 1 .
         [Relationship F1]: D 1 ≦D 2 ≦ . . . ≦Dn, and D 1 &lt;Dn       

     A value (R/L) obtained by dividing the loft R (degree) by the length L (inch) is defined as X. A product of the depth D (mm) and the height H (mm) is defined as Y. A club having a shorter length L has a greater X. 
     The number of clubs of the set A1 is defined as N (N is an integer of equal to or greater than 2), and Y of the clubs are defined as Y 1 , Y 2 , . . . , Yn in an order from the club having the smallest X, the set A1 satisfies the following relationship F2.
         [Relationship F2]: Y 1 ≦Y 2 ≦ . . . ≦Yn, and Y 1 &lt;Yn       

     The set A includes a club set A2 of HB category. The set A2 includes a plurality of clubs belonging to HB category. Specifically, the set A2 includes four clubs belonging to HB category. 
     In the set A2, the greater the real loft R is, the shorter the club length L is. In the set A2, the heights H are constant. In the set A2, the depths D are varied. In the set A2, as the club length L becomes shorter, the depth D tends to be greater. 
     The number of clubs of the set A2 is defined as M (M is an integer of equal to or greater than 2), and the recess-part depths D of the clubs are defined as D 1 , D 2 , . . . , Dm in an order from the club having the greatest length L, the set A2 satisfies the following relationship H1.
         [Relationship H1]: D 1 ≦D 2 ≦ . . . ≦Dm, and D 1 &lt;Dm       

     The number of clubs of the set A2 is defined as M (M is an integer of equal to or greater than 2), and Y of the clubs are defined as Y 1 , Y 2 , . . . , Ym in an order from the club having the smallest X, the set A2 satisfies the following relationship H2.
         [Relationship H2]: Y 1 ≦Y 2 ≦ . . . ≦Ym, and Y 1 &lt;Ym       

     Thus, the embodiment 1 discloses the golf club set A including at least one club of FW category and at least one club of HB category. The set A includes a plurality of clubs of FW category. The set A includes a plurality of clubs of HB category. At least two clubs of the set A can be a golf club set in the present application. For example, the set A1 is a golf club set in the present application. The set A2 is also a golf club set in the present application. At least two clubs of the set A1 can be a golf club set in the present application. At least two clubs of the set A2 can be a golf club set in the present application. 
     The embodiment 1 discloses the set A1 of FW category which satisfies the relationship F1. The embodiment 1 discloses the set A1 of FW category which satisfies the relationship F2. The embodiment 1 discloses the set A2 of HB category which satisfies the relationship H1. The embodiment 1 discloses the set A2 of HB category which satisfies the relationship H2. 
     Table 2 below shows specifications of a club set B of a second embodiment. The set B includes a club of FW category and a club of HB category. The set B includes a club having a depth D of zero. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Table 2 Specifications of set B 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Recess- 
                   
                   
               
               
                   
                   
                 Real 
                 Club 
                   
                 part  
                   
                   
               
               
                   
                   
                 loft  
                 length  
                 Height 
                 depth  
                   
                   
               
               
                   
                   
                 R 
                 L 
                 H 
                 D 
                 X 
                 Y 
               
               
                   
                   
                 (degree) 
                 (inch) 
                 (mm) 
                 (mm) 
                 (R/L) 
                 (D × H) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Set B1 
                 #3 
                 15 
                 43 
                 1 
                 0 
                 0.35 
                 0.00 
               
               
                 (FW 
                 #4 
                 17 
                 42.75 
                 1.5 
                 0 
                 0.40 
                 0.00 
               
               
                 category) 
                 #5 
                 19 
                 42.5 
                 1.5 
                 1 
                 0.45 
                 1.50 
               
               
                   
                 #7 
                 21 
                 42 
                 1.5 
                 2 
                 0.50 
                 3.00 
               
               
                 Set B2 
                 #2 
                 16 
                 40.75 
                 1.5 
                 2 
                 0.39 
                 3.00 
               
               
                 (HB 
                 #3 
                 19 
                 40.25 
                 1.5 
                 3 
                 0.47 
                 4.50 
               
               
                 category) 
                 #4 
                 22 
                 39.75 
                 1.5 
                 4 
                 0.55 
                 6.00 
               
               
                   
                 #5 
                 25 
                 39.25 
                 1.5 
                 4 
                 0.64 
                 6.00 
               
               
                   
               
            
           
         
       
     
     The set B includes a club set B1 of FW category. The set B1 includes a plurality of clubs belonging to FW category. Specifically, the set B1 includes four clubs belonging to FW category. 
     In the set B1, the greater the real loft R is, the shorter the club length L is. In the set B1, the heights H are varied. 
     When the number of clubs of the set B1 is defined as N (N is an integer of equal to or greater than 2), and the heights H of the clubs are defined as H 1 , H 2 , . . . , Hn in an order from the club having the greatest length L, the set B1 satisfies the following relationship F3.
         [Relationship F3]: H 1 ≦H 2 ≦ . . . ≦Hn, and H 1 &lt;Hn       

     In the set B1, the depths D are varied. The set B1 satisfies the relationship F1. The set B1 satisfies the relationship F2. 
     The set B includes a club set B2 of HB category. The set B2 includes a plurality of clubs belonging to HB category. Specifically, the set B2 includes four clubs belonging to HB category. 
     In the set B2, the greater the real loft R is, the shorter the club length L is. In the set B2, the heights H are constant. In the set B2, the depths D are varied. The set B2 satisfies the relationship H 1 . The set B2 satisfies the relationship H2. 
     Thus, the embodiment 2 discloses the golf club set B including at least one club of FW category and at least one club of HB category. The set B includes a plurality of clubs of FW category. The set B includes a plurality of clubs of HB category. At least two clubs of the set B can be a golf club set in the present application. For example, the set B1 is a golf club set in the present application. The set B2 is a golf club set in the present application. At least two clubs of the set B1 can be a golf club set in the present application. At least two clubs of the set B2 can be a golf club set in the present application. 
     When the number of clubs of the set B is defined as Q (Q is an integer of equal to or greater than 2), and the depths D of the clubs are defined as D 1 , D 2 , . . . , Dq in an order from the club having the greatest length L, the set B satisfies the following relationship FH1.
         [Relationship FH1]: D 1 ≦D 2 ≦ . . . ≦Dq, and D 1 &lt;Dq       

     When the number of clubs of the set B is defined as Q (Q is an integer of equal to or greater than 2), and Y of the clubs are defined as Y 1 , Y 2 , . . . , Yq in an order from the club having the greatest length L, the set B satisfies the following relationship FH2.
         [Relationship FH2]: Y 1 ≦Y 2 ≦ . . . ≦Yq, and Y 1 &lt;Yq       

     The embodiment 2 discloses the set B1 of FW category which satisfies the relationship F1. The embodiment 2 discloses the set B1 of FW category which satisfies the relationship F2. The embodiment 2 discloses the set B1 of FW category which satisfies the relationship F3. The embodiment 2 discloses the set B2 of HB category which satisfies the relationship H1. The embodiment 2 discloses the set B2 of HB category which satisfies the relationship H2. 
     Table 3 below shows specifications of a club set C of a third embodiment. The set C includes a driver, a club of FW category and a club of HB category. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Table 3 Specifications of set C 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Recess- 
                   
                   
               
               
                   
                   
                 Real 
                 Club 
                   
                 part  
                   
                   
               
               
                   
                   
                 loft  
                 length  
                 Height 
                 depth  
                   
                   
               
               
                   
                   
                 R 
                 L 
                 H 
                 D 
                 X 
                 Y 
               
               
                   
                   
                 (degree) 
                 (inch) 
                 (mm) 
                 (mm) 
                 (R/L) 
                 (D × H) 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Driver 
                 #1 
                 8.5 
                 44.75 
                 1 
                 1 
                 0.19 
                 1.00 
               
               
                 Set C1 
                 #3 
                 15 
                 43 
                 1.5 
                 2 
                 0.35 
                 3.00 
               
               
                 (FW 
                 #4 
                 17 
                 42.75 
                 1.5 
                 2 
                 0.40 
                 3.00 
               
               
                 category) 
                 #5 
                 19 
                 42.5 
                 1.5 
                 3 
                 0.45 
                 4.50 
               
               
                   
                 #7 
                 21 
                 42 
                 1.5 
                 3 
                 0.50 
                 4.50 
               
               
                 Set C2 
                 #2 
                 16 
                 40.75 
                 1.5 
                 4 
                 0.39 
                 6.00 
               
               
                 (HB 
                 #3 
                 19 
                 40.25 
                 1.5 
                 4 
                 0.47 
                 6.00 
               
               
                 category) 
                 #4 
                 22 
                 39.75 
                 1.5 
                 5 
                 0.55 
                 7.50 
               
               
                   
                 #5 
                 25 
                 39.25 
                 3 
                 5 
                 0.64 
                 15.00 
               
               
                   
               
            
           
         
       
     
     The set C includes a driver. A typical driver satisfies the following specifications (3a) to (3e).
     (3a) The head has a curved face surface.   (3b) The head has a hollow part.   (3c) The head has a volume of greater than 300 cc but equal to or less than 470 cc.   (3d) The head has a real loft of equal to or greater than 6 degrees but equal to or less than 15 degrees.   (3e) The club length is equal to or longer than 43.5 inches but equal to or shorter than 48 inches.   

     The set C includes a club set C1 of FW category. The set C1 includes a plurality of clubs belonging to FW category. Specifically, the set C1 includes four clubs belonging to FW category. 
     In the set C1, the greater the real loft R is, the shorter the club length L is. In the set C1, the heights H are varied. The set C1 satisfies the relationship F3. 
     In the set C1, the depths D are varied. The set C1 satisfies the relationship F1. The set C1 satisfies the relationship F2. 
     The set C includes a club set C2 of HB category. The set C2 includes a plurality of clubs belonging to HB category. Specifically, the set C2 includes four clubs belonging to HB category. 
     In the set C2, the greater the real loft R is, the shorter the club length L is. In the set C2, the heights H are varied. 
     When the number of clubs of the set C2 is defined as M (M is an integer of equal to or greater than 2), and the heights H of the clubs are defined as H 1 , H 2 , . . . Hm in an order from the club having the greatest length L, the set C2 satisfies the following relationship H3.
         [Relationship H3]: H 1 ≦H 2 ≦ . . . ≦Hm, and H 1 &lt;Hm       

     In the set C2, the depths D are varied. The set C2 satisfies the relationship H1. The set C2 satisfies the relationship H2. 
     Thus, the embodiment 3 discloses the golf club set C including a driver, at least one club of FW category, and at least one club of HB category. The set C includes one driver. The set C includes a plurality of clubs of FW category. The set C includes a plurality of clubs of HB category. At least two clubs of the set C can be a golf club set in the present application. At least two clubs of the set C1 can be a golf club set in the present application. At least two of the set C2 can be a golf club set in the present application. At least two clubs including a driver and at least one club belonging to FW category or belonging to HB category can be a golf club set in the present application. 
     The set C satisfies the relationship FH1. The set C satisfies the relationship FH2. 
     The embodiment 3 discloses the set C including a driver and a club of FW category and/or a club of HB category. In the set C, the height H of the driver is the minimum in comparison between the heights H of the clubs in the set. In the set C, the depth D of the driver is the minimum in comparison between the depths D in the set. 
     The embodiment 3 discloses the set C1 of FW category which satisfies the relationship F1. The embodiment 3 discloses the set C1 of FW category which satisfies the relationship F2. The embodiment 3 discloses the set C1 of FW category which satisfies the relationship F3. The embodiment 3 discloses the set C2 of HB category which satisfies the relationship H1. The embodiment 3 discloses the set C2 of HB category which satisfies the relationship H2. The embodiment 3 discloses the set C2 of HB category which satisfies the relationship H3. 
     A request to a club for performance capable of directly aiming at a target (pin) is increased as the club length L of the club becomes shorter. Therefore, it is preferable that a higher launch angle and more backspin are achieved as the club length becomes shorter. By increasing the depth D, the height H, or Y, deformation of the crown is promoted, and thus deformation of increasing the loft angle is likely to occur. This deformation can attain the higher launch angle and more backspin. In these respects, in a set of FW category, a set satisfying the relationship F1 is preferable, a set satisfying the relationship F2 is preferable, and a set satisfying the relationship F3 is preferable. In the same respects, in a set of the HB category, a set satisfying the relationship H1 is preferable, a set satisfying the relationship H2 is preferable, and a set satisfying the relationship H3 is preferable. In the same respects, a set satisfying the relationship FH1 is preferable, and a set satisfying the relationship FH2 is preferable. 
     Clubs which constitute a set might be purchasable as each single club. However, a group of clubs having respective club numbers different from each other and belonging to the same product class is regarded as a set. Usually, two or more clubs belonging to the group of clubs are purchased at the same time. Therefore, even if clubs which constitute a set might be purchasable as each single club, such a selling is substantially regarded as a selling of a set. 
     As mentioned above, the index X and the index Y are considered in the present application. The index X is a value obtained by dividing the loft R (degree) by the length L (inch). In other words, X=R/L. The index Y is a product of the depth D (mm) and the height H (mm). In other words, Y=D×H. 
     As described above, each club has a required performance depending on the club length L (loft angle R). Depending on each club number, a balance between performance for directly aiming at a target and flight distance performance is required. In this respect, when the loft R is great and the length L is small, the index Y is preferably great. That is, as the index X is increased, the index Y is preferably also increased. In this respect, the following [Specification 1] is preferable, and the following [Specification 2] is more preferable.
         [Specification 1]: a golf club having X of equal to or greater than 0.1 but equal to or less than 0.9, and having Y of greater than 0 but equal to or less than 25   [Specification 2]: a golf club having X of equal to or greater than 0.25 but equal to or less than 0.8, and having Y of greater than 0 but equal to or less than 25       

     A request to a club of HB category for directly aiming at a target is further stronger as compared with a club of FW category. In this respect, the categories can adopt respective specifications different from each other. In view of a club of FW category, the following [Specification 3] is more preferable. In view of a club of HB category, the following [Specification 4] is more preferable.
         [Specification 3]: a golf club having X of equal to or greater than 0.25 but equal to or less than 0.7, and having Y of greater than 0 but equal to or less than 10   [Specification 4]: a golf club having X of equal to or greater than 0.3 but equal to or less than 0.8, and having Y of equal to or greater than 4 but equal to or less than 10       

     In view of optimization for each club number, the depth D is preferably as follows.
     (1) In a golf club belonging to FW category and having a real loft R of equal to or less than 17°, the depth D is preferably equal to or greater than 0 mm, and more preferably equal to or greater than 1 mm, but preferably equal to or less than 3 mm, and more preferably equal to or less than 2 mm.   (2) In a golf club belonging to FW category and having a real loft R of greater than 17°, the depth D is preferably equal to or greater than 1 mm, and more preferably equal to or greater than 2 mm, but preferably equal to or less than 4 mm, and more preferably equal to or less than 3 mm.   (3) In a golf club belonging to HB category and having a real loft R of equal to or less than 19°, the depth D is preferably equal to or greater than 1 mm, more preferably equal to or greater than 2 mm, and still more preferably equal to or greater than 3 mm, but preferably equal to or less than 5 mm, and more preferably equal to or less than 4 mm.   (4) In a golf club belonging to HB category and having a real loft R of greater than 19°, the depth D is preferably equal to or greater than 2 mm, and more preferably equal to or greater than 3 mm, but preferably equal to or less than 6 mm, and more preferably equal to or less than 5 mm.   

     In view of optimization for each club number, the index Y is preferably as follows.
     (1) In a golf club belonging to FW category and having a real loft R of equal to or less than 17°, Y is preferably equal to or greater than 0, and more preferably equal to or greater than 1, but preferably equal to or less than 4, and more preferably equal to or less than 3.   (2) In a golf club belonging to FW category and having a real loft R of greater than 17°, Y is preferably equal to or greater than 1, and more preferably equal to or greater than 1.5, but preferably equal to or less than 6, and more preferably equal to or less than 5.   (3) In a golf club belonging to HB category and having a real loft R of equal to or less than 19°, Y is preferably equal to or greater than 1, more preferably equal to or greater than 1.5, and still more preferably equal to or greater than 2, but preferably equal to or less than 7, and more preferably equal to or less than 6.   (4) In a golf club belonging to HB category and having a real loft R of greater than 19°, Y is preferably equal to or greater than 2, more preferably equal to or greater than 3, and still more preferably equal to or greater than 4, but preferably equal to or less than 17, and more preferably equal to or less than 15.   

     EXAMPLES 
     Hereinafter, effects of the present invention will become apparent according to examples. However, the present invention should not be restrictively construed based on the description of examples. 
     Example 1 
     Recess Part in the Front-back Direction 
     The same head as the head depicted in  FIGS. 28 to 32  was produced. A titanium alloy was used as the material of the head. A head body except a face part was produced by lost-wax precision casting. A face member was produced by forging. The obtained head body and the face member were welded to obtain a head of Example 1. 
     Example 2 
     Recess Part in the Toe-heel Direction 
     The same head as the head depicted in  FIGS. 1 to 6  was produced. A titanium alloy was used as the material of the head. A head body except a face part was produced by lost-wax precision casting. A face member was produced by forging. The obtained head body and the face member were welded to obtain a head of Example 2. 
     Examples 3 to 5 
     Heads of Examples 3 to 5 were obtained in the same manner as Example 2 except the height H was changed. 
     Comparative Example 
     The head of Comparative Example was obtained in the same manner as Example 2 except a recess part and a step surface were not provided. 
     Example 6 
     The head of Example 6 was obtained in the same manner as Example 2 except the height H and the depth D were changed. 
     Examples 7 to 11 
     Heads of Examples 7 to 11 were obtained in the same manner as Example 6 except the depth D was changed. 
     Examples 12 to 16 
     Heads of Examples 12 to 16 were obtained in the same manner as Example 7 except the distance T was changed. 
     Specifications and evaluation results of Examples and Comparative Example are shown in Tables 4 to 6 below. The evaluation results are shown by difference from Comparative Example. When the difference from Comparative Example is slight, the evaluated value is omitted. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Table 4 Specifications and Evaluation results of  
               
               
                 Examples and Comparative Example 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Comp. 
                   
                   
                   
                   
                   
               
               
                   
                 Ex. 
                 Ex. 1 
                 Ex. 2 
                 Ex. 3 
                 Ex. 4 
                 Ex. 5 
               
               
                   
               
               
                 Extending 
                 no step 
                 toe- 
                 toe- 
                 toe- 
                 toe- 
                 toe- 
               
               
                 direction of 
                 surface 
                 heel 
                 heel 
                 heel 
                 heel 
                 heel 
               
               
                 step surface 
                   
                 direc- 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
               
               
                   
                   
                 tion 
                 tion 
                 tion 
                 tion 
                 tion 
               
               
                 Height H (mm) 
                 — 
                 1 
                 1 
                 2 
                 3 
                 4 
               
               
                 Extending 
                 no 
                 front- 
                 toe- 
                 toe- 
                 toe-  
                 toe- 
               
               
                 direction of 
                 recess 
                 back 
                 heel 
                 heel 
                 heel 
                 heel 
               
               
                 recess part 
                 part 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
               
               
                   
                   
                 tion 
                 tion 
                 tion 
                 tion 
                 tion 
               
               
                 Depth D (mm) 
                 — 
                 1 
                 2 
                 2 
                 2 
                 2 
               
               
                   
                   
                 (Maxi- 
                   
                   
                   
                   
               
               
                   
                   
                 mum 
                   
                   
                   
                   
               
               
                   
                   
                 value) 
                   
                   
                   
                   
               
               
                 Distance T 
                 — 
                 10 
                 10 
                 10 
                 10 
                 10 
               
               
                 (mm) 
                   
                   
                   
                   
                   
                   
               
               
                 Backspin 
                 — 
                 10 
                 50 
                 100 
                 130 
                 150 
               
               
                 (rpm: against 
                   
                   
                   
                   
                   
                   
               
               
                 Comp. Ex.) 
                   
                   
                   
                   
                   
                   
               
               
                 Launch angle 
                 — 
                 0.1 
                 0.1 
                 0.2 
                 0.3 
                 0.4 
               
               
                 (degree: against  
                   
                   
                   
                   
                   
                   
               
               
                 Comp. Ex.) 
                   
                   
                   
                   
                   
                   
               
               
                 Coefficient 
                 — 
                 — 
                 — 
                 0.001 
                 0.002 
                 0.003 
               
               
                 of restitution 
                   
                   
                   
                   
                   
                   
               
               
                 (Against 
                   
                   
                   
                   
                   
                   
               
               
                 Comp. Ex.) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Table 5 Specifications and evaluation results of Examples  
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Ex. 6 
                 Ex. 7 
                 Ex. 8 
                 Ex. 9 
                 Ex. 10 
                 Ex. 11 
               
               
                   
               
               
                 Extending 
                 toe- 
                 toe- 
                 toe- 
                 toe- 
                 toe- 
                 toe- 
               
               
                 direction of 
                 heel 
                 heel 
                 heel 
                 heel 
                 heel 
                 heel 
               
               
                 step surface 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
               
               
                   
                 tion 
                 tion 
                 tion 
                 tion 
                 tion 
                 tion 
               
               
                 Height H (mm) 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
               
               
                 Extending 
                 toe- 
                 toe- 
                 toe- 
                 toe- 
                 toe-  
                 toe- 
               
               
                 direction of 
                 heel 
                 heel 
                 heel 
                 heel 
                 heel 
                 heel 
               
               
                 recess part 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
               
               
                   
                 tion 
                 tion 
                 tion 
                 tion 
                 tion 
                 tion 
               
               
                 Depth D (mm) 
                 1 
                 2 
                 3 
                 2 
                 5 
                 6 
               
               
                 Distance T 
                 10 
                 10 
                 10 
                 10 
                 10 
                 10 
               
               
                 (mm) 
                   
                   
                   
                   
                   
                   
               
               
                 Backspin 
                 20 
                 80 
                 120 
                 150 
                 160 
                 170 
               
               
                 (rpm: against 
                   
                   
                   
                   
                   
                   
               
               
                 Comp. Ex.) 
                   
                   
                   
                   
                   
                   
               
               
                 Launch angle 
                 0.1 
                 0.2 
                 0.3 
                 0.5 
                 0.6 
                 0.7 
               
               
                 (degree: against  
                   
                   
                   
                   
                   
                   
               
               
                 Comp. Ex.) 
                   
                   
                   
                   
                   
                   
               
               
                 Coefficient 
                 — 
                 0.001 
                 0.002 
                 0.003 
                 0.003 
                 0.004 
               
               
                 of restitution 
                   
                   
                   
                   
                   
                   
               
               
                 (Against 
                   
                   
                   
                   
                   
                   
               
               
                 Comp. Ex.) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Table 6 Specifications and Evaluation results of Examples  
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Ex. 
                 Ex. 
                 Ex. 
                 Ex.  
                 Ex. 
                 Ex. 
               
               
                   
                 12 
                 13 
                 14 
                 7 
                 15 
                 16 
               
               
                   
               
               
                 Extending 
                 toe- 
                 toe- 
                 toe- 
                 toe- 
                 toe- 
                 toe- 
               
               
                 direction of 
                 heel 
                 heel 
                 heel 
                 heel 
                 heel 
                 heel 
               
               
                 step surface 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
               
               
                   
                 tion 
                 tion 
                 tion 
                 tion 
                 tion 
                 tion 
               
               
                 Height H (mm) 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
                 1.5 
               
               
                 Extending 
                 toe- 
                 toe- 
                 toe- 
                 toe- 
                 toe-  
                 toe- 
               
               
                 direction of 
                 heel 
                 heel 
                 heel 
                 heel 
                 heel 
                 heel 
               
               
                 recess part 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
                 direc- 
               
               
                   
                 tion 
                 tion 
                 tion 
                 tion 
                 tion 
                 tion 
               
               
                 Depth D (mm) 
                 2 
                 2 
                 2 
                 2 
                 2 
                 2 
               
               
                 Distance T 
                 3 
                 5 
                 7 
                 10 
                 12 
                 15 
               
               
                 (mm) 
                   
                   
                   
                   
                   
                   
               
               
                 Backspin 
                 10 
                 30 
                 60 
                 80 
                 20 
                 20 
               
               
                 (rpm: against 
                   
                   
                   
                   
                   
                   
               
               
                 Comp. Ex.) 
                   
                   
                   
                   
                   
                   
               
               
                 Launch angle 
                 — 
                 0.1 
                 0.2 
                 0.2 
                 0.2 
                 0.2 
               
               
                 (degree: against  
                   
                   
                   
                   
                   
                   
               
               
                 Comp. Ex.) 
                   
                   
                   
                   
                   
                   
               
               
                 Coefficient 
                 — 
                 — 
                 — 
                 0.001 
                 0.001 
                 0.002 
               
               
                 of restitution 
                   
                   
                   
                   
                   
                   
               
               
                 (Against 
                   
                   
                   
                   
                   
                   
               
               
                 Comp. Ex.) 
                   
                   
                   
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     Methods for Evaluations are as follows. 
     [Launch Angle and Backspin] 
     A golf ball was hit by a swing robot, and the launch angle and backspin were measured. The hitting was made five times for each golf club. The hitting point was set to the face center. Average values of all data were calculated. Differences from Comparative Example are shown in Tables 4 to 6. 
     [Coefficient of Restitution (COR)] 
     Coefficient of restitution was measured for each head based on Procedure for Measuring the Velocity Ratio of a Club Head for Conformance to Rule 4-1e, Revision 2 (Feb. 8, 1999) issued by U.S.G.A. Coefficient of restitution at the sweet spot was measured. Differences from Comparative Example are shown in Tables 4 to 6. 
     As shown in Tables, Examples have higher evaluations than that of Comparative Example. Examples have more backspin, a greater launch angle and a greater coefficient of restitution. Examples have an excellent function, as a club, of directly aiming at a target. From these results, advantages of the present invention are clear. 
     The present invention can be applied to all golf club heads such as a wood type head, a hybrid type head, and an iron type head. Preferably, the present invention can applied to a wood type head and a hybrid type head. 
     The description hereinabove is merely for an illustrative example, and various modifications can be made in the scope not to depart from the principles of the present invention.