You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     Fences comprised of vertically oriented pickets attached by one or more rails are commonly used. Typically, each rail is attached to multiple pickets with screws. This provides a secure attachment which allows the rail to pivot about the screw axis. Therefore, such a fence does have a limited ability to be used on an incline because the angle between the pickets and the rail may be adjusted. However, the adjustability of the angle is limited by the fact that the screw attaches the rail to the pickets. Further, the screw heads are exposed. This is not aesthetically pleasing. 
     What is needed is a device which allows the screws to be covered. Such a device does exist. For example, see U.S. Pat. No. 6,752,385 issued to Zen et al. Although the screw heads are covered in that device, the adjustability of the angle between the pickets and the rail remains limited because the inner rail is directly attached to the pickets. What is also needed is a fence assembly wherein the rail may be quickly and easily installed onto and removed from the pickets. Ideally, the rail and the pickets would not be pivotally connected to each other. This would allow greater adjustability to the angle between the rail and the pickets. Such greater angle adjustability permits the fence assembly to be used on a larger variety of terrains and inclines. Preferably, the pickets would be attached to each other with an inner rail and an outer rail would snap fit over the inner rail. The outer rail should be able to slide relative to the inner rail. 
     SUMMARY OF THE PRESENT INVENTION 
     The fence assembly described herein is comprised of a plurality of pickets, an inner rail and an outer rail. The pickets are traditional fence pickets. 
     The inner rail is a strip comprising an upper locking seat, a lower locking seat and a fastener recess. The fastener recess is positioned between the seats. The upper locking seat has an upper locking seat horizontal member and an upper locking seat vertical member. The lower locking seat has a vertical base and a lower locking seat horizontal member extending from the vertical base. The horizontal members form two walls which define walls of the recess. The lower wall is longer than the upper wall. The depth of the recess is sufficient to cover the head of a fastener positioned within the recess when the fastener is used to secure the inner rail to a picket. The inner rail is fastened to the plurality of pickets with one or more fasteners. The head of each fastener is enclosed within the recess. 
     The outer rail has an upper locking tang, a lower locking tang and a tensioning wall. The upper locking tang has an upper locking tang horizontal member and an upper locking tang vertical member. The upper locking tang vertical member extends vertically from the upper locking tang horizontal member. The upper locking tang vertical member is sized and shaped to fit adjacent to the upper locking seat vertical member such that horizontal movement of the outer rail is limited by contact between the upper locking tang vertical member and the upper locking seat vertical member. The upper locking tang horizontal member is sized and shaped to fit above the upper locking seat vertical member such that vertical movement of the outer rail is limited by contact between the upper locking tang horizontal member and the upper locking seat vertical member. The lower locking tang is sized and shaped to fit adjacent to the vertical base and the horizontal member of the lower locking seat such that horizontal and vertical movement between the lower locking tang of the outer rail and the lower locking seat is limited. The tensioning wall is adapted to maintain contact with a picket extending into the outer rail. 
     The outer rail snap fits over the inner rail to secure the outer rail to the pickets. Horizontal and vertical movement between the outer rail and the pickets is limited. The angle between the outer rail and the pickets is adjustable. The heads of the fasteners within the recess, which attach the inner rail to the pickets, are covered by the outer rail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  is a perspective view of a fence assembly. 
         FIG. 2  is an exploded perspective view of a part of the fence assembly of  FIG. 1 . 
         FIG. 3  is a partial exploded perspective view of a part of the fence assembly of  FIG. 1 , wherein the inner rail of the fence assembly is secured to a picket with a screw. 
         FIG. 4  is a sectional side view of the fence assembly of  FIG. 1  showing the relationship between the outer rail, the inner rail and a picket. 
         FIG. 5  is a sectional side view of an alternate embodiment of the inner rail of  FIG. 3 . 
         FIG. 6A  is a front view of a prior art fence construction showing an outer rail perpendicularly mounted to a picket. 
         FIG. 6B  is a front view of a prior art fence construction showing an outer rail angled with respect to a picket, wherein the angle is limited by the pivot point resulting from the attaching screw and the opening within the outer rail. 
         FIG. 6C  is a front view of a section of the fence assembly described herein, wherein the outer rail is installed upon the inner rail and a picket extends through the outer rail. 
         FIG. 6D  is a front view of this section of the fence assembly shown in  FIG. 6  wherein the outer rail is maximally angled with respect to the picket. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred fence assembly is comprised of a plurality of pickets  20 , an inner rail  22  and an outer rail  42 . The pickets  20 , inner rail  22  and outer rail  42  are configured to form a fence assembly. 
     The pickets  20  are conventional fence pickets. In a fence assembly they are vertically oriented. Typically, the pickets  20  have a rectangular or square cross-section. They may be extruded from aluminum. The inner rail  22  and the outer rail  42  may also be extruded from aluminum. Aluminum is extrudable, light weight, has a relatively low cost and is resistant to oxidation. 
     The pickets  20  in a fence assembly are secured in a vertical configuration by an inner rail  22  fastened to the pickets  20 . The inner rail  22  has an upper locking seat  24 , a lower locking seat  30  and a fastener recess  36  positioned between the locking seats  24 ,  30 . The upper locking seat  24  has an upper locking seat horizontal member  26  and an upper locking seat vertical member  28 , as shown in  FIG. 4 . The upper locking seat horizontal member  26  and the upper locking seat vertical member  28  form an upper wall projecting from the inner rail  22 . The upper locking seat vertical member  28  is a substantially vertical projection from this wall positioned on the distal end of the upper locking seat horizontal member  26 . Preferably, the distal end of the upper locking seat vertical member  28  is at a clockwise acute angle with respect to the upper locking seat horizontal member  26 . This upper wall has a shorter length than the lower wall of the inner rail  22  described below. The lower locking seat  30  has a vertical base  32  and a lower locking seat horizontal member  34 . The lower locking seat horizontal member  34  extends from the vertical base of  32 , as shown in  FIG. 4 . The vertical base  32  of the lower locking seat  30  is provided with an indentation for receiving the lower locking tang  50  of an outer rail  42 , as shown in  FIG. 4 . The top of the indentation is defined by the bottom surface of the lower locking seat horizontal member  34 . The side of the indentation is defined by a vertical portion of the vertical base  32  of the lower locking seat  30 . This vertical portion is preferably acutely angled with respect to the vertical direction and the bottom portion of the vertical base  32  of the lower locking seat  30 , as shown in  FIG. 4 . As a result, the lower locking tang  50  will snap into and be secured by the indentation. The horizontal member  34  of the lower locking seat  30  forms a lower wall projecting from the inner rail  22 . Thus, the horizontal member  26  of the upper locking seat  24  and the horizontal member  34  of the lower locking seat  30  form two walls on the inner rail  22 . The walls of the inner rail  22  define a recess  36 . The fasteners  38  are positioned within the recess  36  in order to secure the inner rail  22  to a picket  20 . The recess  36  has sufficient depth such that the head  40  of a fastener  38 , such as a screw, positioned within the recess  36  to secure the inner rail  22  to a picket  20 , is enclosed within the recess  36 . This facilitates the covering of the fastener head  40  by the outer rail  42  without the fastener head  40  coming into contact with the outer rail  42 . The lower seat horizontal member  34  forms a longer wall than the wall formed by the upper seat horizontal member  26 , as shown in  FIG. 4 . This will facilitate installation (by snapping) and removal of the outer rail  42  from the inner rail  22 . The inner rail  22  is fastened to the plurality of pickets  20  with one or more fasteners  38 . The heads  40  of each fastener  38  are enclosed within the recess  36 . 
     The outer rail  42  has an upper locking tang  44 , a lower locking tang  50  and a tensioning wall  54 , as shown in  FIG. 4 . The upper locking tang  44  has an upper locking tang horizontal member  46  and an upper locking tang vertical member  48  extending from the upper locking tang horizontal member  46 . This is shown in  FIG. 4 . The upper left portion of that figure shows the upper locking tang  44 . The upper locking tang  44  has a horizontal member  46 . The upper locking tang vertical member  48  extends vertically from the upper locking tang horizontal member  46 . Reference directions such as vertical and horizontal refer to directions relative to a traditional fence. When an outer rail  42  is snapped over an inner rail  22  the upper locking tang vertical member  48  is trapped into a locked positioned by the upper locking seat vertical member  28 , as shown in  FIG. 4 . The pickets are vertically oriented. The rails are horizontally oriented. The reference directions are not absolute. For example, when a fence is installed upon a hill the pickets may be vertically oriented and the rails horizontally oriented with respect to the hill, but not so oriented with respect to a flat non-inclined part of the ground. Vertically oriented and horizontally oriented mean substantially vertically oriented and substantially horizontally oriented, respectively. The upper locking tang horizontal member  46 , the upper locking tang vertical member  48  and the upper portion of the tensioning wall  54  form a U-shaped configuration. The upper locking tang vertical member  48  is sized and shaped to fit adjacent to the upper locking seat vertical member  28 , as shown in  FIG. 4 , such that horizontal movement of the outer rail  42  is limited by contact between the upper locking tang vertical member  48  and the upper locking seat vertical member  28 . The upper locking tang horizontal member  46  is sized and shaped to fit above the upper locking seat vertical member  28 , as shown in  FIG. 4 , such that vertical movement of the outer rail  42  is limited by contact between the upper locking tang horizontal member  46  and the upper locking seat vertical member  28 . The lower locking tang  50  is sized and shaped to fit adjacent to the vertical base  32  and the horizontal member  34  of the lower locking seat  30 , as shown in  FIG. 4 , such that horizontal and vertical movement between the lower locking tang  50  of the outer rail  42  and the lower locking seat  30  is limited. Preferably, the lower locking tang  50  bottom surface is beveled with a convex contour, as shown in  FIG. 4 . This will cause the lower locking tang  50  to snap over the horizontal member  34  of the lower locking seat  30  when downward pressure is applied to the outer rail  42  while the lower surface of the lower locking tang  50  is in contact with the upper surface of the lower locking seat horizontal member  34 . This occurs during installation. Preferably, the lower locking tang  50  is notched and angled to fit within the lower locking seat  30  of the inner rail  22 , as shown in  FIG. 4 . The angle of the proximal end of the lower surface of the lower locking tang  50  preferably matches the previously described angle formed on the vertical base  32  of the inner rail  22 . The tensioning wall  54  is adapted to maintain contact with a picket  20  extending into the outer rail  42 . As shown in  FIG. 4 , the tensioning wall  54  is an L-shaped wall extending vertically from the outer rail  42 . The tensioning wall  54  is adapted to impart a springlike force upon a picket  20 . The interaction between the outer rail  42  and the inner rail  22  when the outer rail  42  is snapped over the inner rail  22  is such that the outer rail  42  is securely attached to the inner rail  22 . Horizontal and vertical movement between the outer rail  42  and the pickets  20  is limited. However, the angle between the outer rail  42  and the pickets  20  is adjustable, as shown and  FIG. 6D . 
     The upper surface of an outer rail  42  may be a closed surface if the outer rail  42  is the upper rail of a fence. In other words, the pickets  20  may or may not extend through the upper outer rail  42 . However, outer rails  42  below the top outer rail  42  must have openings  52 . The openings  52  are necessary in order for the pickets  20  to extend through the outer rails  42 . The outer rails  42  may or may not have an enclosed lower surface. An enclosed lower surface on an outer rail  42  results in that rail having four walls forming a rectangular structure. In this case the bottom wall of the outer rail  42  must have an opening for a picket  20  to extend into that outer rail  42 . The preferred embodiment described and illustrated in  FIG. 4  allows the outer rail  42  to slide along the inner rail  22 . This is illustrated in  FIGS. 6C and 6D . There, a picket opening  52  is illustrated in the upper surface of an outer rail  42 . The opening  52  is larger than the picket  20 . This allows the outer rail  42  to be angled with respect to the picket  20 , as shown in  FIG. 6D . This angling has a very practical application. When a fence is installed on an incline such as a hill the outer rail members  42  and the pickets  20  are not perpendicular to each other. A perpendicular orientation is shown in  FIG. 6C . It is important for a fence installer to be able to angle the outer rails  42  with respect to the pickets  20  in order to properly install a fence on an incline. As shown in  FIG. 6D , the angle can be maximized by utilizing the entire width of the opening  52  within the outer rail  42 . The entire width of the opening  52  of the outer rails  42  of the fence assembly described herein may be utilized, as shown in  FIG. 6D . The entire width may be utilized because the outer rail  42  can slide relative to the inner rail  22 . This should be compared to a traditional picket/outer rail construction, as shown in  FIG. 6A  and  FIG. 6B . In this prior art construction the outer rail is directly secured to the picket with a screw. Although the screw allows the outer rail to pivot, the pivot angle is limited because the outer rail is directly secured to the picket, as shown in  FIG. 6B . 
     In the fence assembly described herein the outer rail  42  snap fits over the inner rail  22 . This secures the outer rail  42  to the pickets  20 . Horizontal and vertical movement between the outer rail  42  and the pickets  20  is limited. However, the angle between the outer rail  42  and the pickets  20  is adjustable because the outer rail  42  may slide along the inner rail  22 . The inner rail  22  is secured to the pickets  20  by fasteners  38 , such as screws. The heads  40  of the fasteners  38  are confined to the recess  36  of the inner rail and they are covered by the outer rail  42 . 
     A similar prior art fence assembly is disclosed in U.S. Pat. No. 7,384,025. The fence assembly described herein has a number of advantages compared to the prior art. The fit between the outer rail  42  and the inner rail  22  and the configuration of those components allows a greater extrusion tolerance with respect to the manufacturing of the outer rails  42  and the inner rails  22 . Greater absolute dimension deviations can be tolerated for the fence assembly described herein before there is a failure of the connection between the outer rail  42  and the inner rail  22 . The tensioning wall  54  tends to keep the pickets  20  properly positioned within the outer rail  42 , as opposed to be being pushed to one side of the outer rail  42 . The downward pressure of the upper locking tang  44  upon the upper locking seat  24  results in a stronger more stable rail. The increased height of the inner rail  22  permitted by the current design enhances the likelihood that the pickets  20  will remain properly aligned with the outer rails  42 . The recess  36  of the inner rail  22  is of sufficient size to receive the heads of Philips pan head screws as well as a variety of other fastener heads. There is an increased ability to angle the outer rail  42  with respect to the pickets  20  because of the ability of the outer rail  42  to slide along the inner rail  22 , as previously described. The fence assembly described has greater strength because it does not rely upon a relatively short upper rib/stopper to properly secure and position the outer rail  42  with respect to the pickets  20 . 
     Although the invention has been shown and described with reference to certain preferred embodiments and methods, those skilled in the art undoubtedly will find alternative embodiments and methods obvious after reading this disclosure. With this in mind, the following claims are intended to define the scope of protection to be afforded the inventor, and those claims shall be deemed to include equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.

Summary:
An improved fence assembly is comprised of a plurality of pickets, an inner rail and an outer rail. The pickets are vertically oriented and attached to the inner rail. The inner rail has an upper locking seat, a lower locking seat and a recess between the seats. The fasteners attaching the inner rail to the pickets have their heads entirely enclosed within the recess. The outer rail snap fits over the inner rail. The outer rail is securely attached to the inner rail, but it may slide along the inner rail. The outer rail has an upper locking tang, a lower locking tang and a tensioning wall. These components slidingly interlock with the inner rail locking seats to provide a fence assembly wherein the outer rail is slidingly attached to the inner rail and the fasteners securing the inner rail to the pickets are hidden.