Abstract:
A unison gauge for indicating levelness of a string set of a grand piano is described. The gauge has a contact block adjustably engageable with the lowermost tangential aspect of the string set. Battery powered indicators provide an indication for each string. With the piano action withdrawn from the piano case, the base of the device rests on the key bed below the piano hammer strike area of the string set and holds the contact block mounted thereon on an extendable mast. The contacts of the contact block are in a plane parallel to the key bed. The contact block is under slight spring tension when engaged with a string set being monitored and, upon compression of the spring readily slides along the key bed to the next string set. A method of use of the unison gauge is also provided.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to a device for determining the levelness of compound strings or string sets, namely bichords and trichords, of a grand piano. The device uses the upper surface of the key bed of the piano as a reference surface and determines levelness of the aforesaid string sets. In addition to the string leveling device, a method of use thereof is provided. 
   2. Description of the Prior Art 
   The history of the modern piano begins with the pianoforte developed by a harpsichord builder, Bartolommeo Christofori. Christofori invented the instrument in Florence in 1709 and completed a prototype around 1720. The main driving force behind the instrument&#39;s development was that, in contradistinction to the predecessor instruments—the harpsichord and the clavichord—the piano provided variations in volume. Additionally development was spurred on by the desire to play fuller, less delicate music. 
   Musicians of the time sought a full rich sound and were unsatisfied by the strong and uniform harpsichord tone produced by plucking large strings with keyboard-controlled quills. On the other hand, the percussive action of the clavichord, while allowing for variations in volume, produced weak tones by striking brass hammers against small, thin strings. 
   Of the several versions of the piano invented in the Eighteenth century, Christofori&#39;s pianoforte, is most closely related to the modern-day grand piano in that the hammers thereof are wooden blocks covered on the striking surface with soft leather. Later in the Eighteenth century a piano hammer was designed with the same basic shape that survives today. In this design, two layers of leather were used with the inner layer being a firm leather, and the outer layer being softer. The final form of the leather covered piano hammer used three layers of leather of varying firmness with the softest leather on the outside and the firmest layer on the inside. This provided a piano hammer that was soft and compliant at the surface to provide the requisite tone for pianissimo playing and a hammer firmer underneath to provide the strength for forte playing. 
   As the grand piano developed, it became more and more a solo instrument, and needed to be louder. To increase volume, strings needed to be thicker and the support structure stronger, so that greater tension could be achieved. The frame of the pianos, commonly made of wood, became thicker and heavier, and was strengthened by cross-bracing. By 1820, English manufacturer John Broadwood began to build iron hitch pin plates, which now meant plates were made of more metal than wood. In 1825 Alpheus Babcock patented the cast-iron frame and further in 1843, American manufacturer Jonas Chickering began making pianos with the full-perimeter plate—a feature of modern grand pianos. 
   In 1821 a French builder, Sebastian Erard, invented the last major basic refinement of the piano action; the double escapement. Shortly thereafter, the heavy iron frame required by the action and the higher tensioned strings was provided, and then cross-stringing, a system whereby the long bass strings cross over the shorter middle-range strings, was invented. 
   After the invention of the iron frame for the strings on a piano, heavier strings, including bichords and trichords, could be used at higher tensions to produce a fuller sound from the piano. This rendered the leather covered piano hammer unacceptable. The result was the development and patenting of a felt covered piano hammer by Alpheus Babcock in 1833. These hammers provided a more acceptable tone than the leather covered hammers. In the late Nineteenth century machines were invented to cover the raw wood piano hammers with felt. The felt hammers enabled the manufacturer to fine tune the tone of the piano by adjusting the hardness of the felt. The process of tonally regulating the piano hammers is called voicing, requires skilled piano technicians, and is a time consuming operation. Initially, the piano hammer felt is checked for proper shaping, particularly that the striking surface is flat across the width thereof. During voicing, the technician adjusts the tone of a hammer by adjusting the softness or the hardness of the hammer felt. When a tone sounds too soft, by applying a solution of lacquer and lacquer thinner to the hammer felt the technician adjusts the felt hardness and the tone produced at that site. Alternate methods of adjusting hammer felt hardness include working the hammer felt with a needle to loosen the fibrous structure thereof. 
   String leveling of a grand piano is a process completed before voicing or the adjustments of the felts of the hammers. If the string sets—bichords and trichords—are not level, the strike of the corresponding hammer produces a fuzzy note and the damping of the strike is not properly accomplished. This creates what is referred to as an aftersound or ring, which, in turn, destroys the normal clarity or definition of individual notes of the instrument. In practice, piano technicians are trained to detect unlevel string sets by ear. Upon detection, the technician is presently taught to correct the condition by withdrawing the piano action and placing the end of a steel rule on the string set to sight along the string. When gaps are present, the sagging string is drawn up to the higher string by shortening or kinking the string. 
   A complete piano action consists of thousands of parts and weighs on the order of 30 kilograms or more and hence has a substantial support or key bed on which the action rests when in the piano case. The key bed is rigidly connected to the piano case and is an integral part of the piano case. Because a mechanical coupling exists between the key bed and the piano soundboard through various components of the piano case, vibrations originating at the key bed are transmitted through the frame of the piano to the soundboard to produce extraneous noise, such as the aftersound, supra, which detract from the tonal integrity of the instrument. Consequently, in a grand piano, as the frame of the piano action is in intimate contact with the key bed, it is critical that the key bed be perfectly flat and that the string sets be precisely level with respect thereto. 
   There are two other aspects of the grand piano manufacture and maintenance that are impacted by string leveling—the una corda pedal action for playing pianissimo and the damping mechanism for curtailing and ending the resonating of a string set. The una corda pedal in the modern grand piano is the left pedal and shifts the piano action sufficiently in the key bed to enable the piano hammers to strike only one string of each bichord unison and two strings of each trichord unison. This places a further requirement on key bed construction to permit a noiseless sliding of the piano action relative to the key bed whenever the una corda pedal is depressed. The description of the una corda pedal and the action thereof is found in a patent to Harold A. Conklin, Jr., U.S. Pat. No. 4,127,051, which description is incorporated herein by reference. 
   In the grand piano, the damping mechanism curtails and ends the resonating of a string or string set. The depressing of a key raises a damper wire which, in turn, supports a damper head and the attached damper felt and thereafter, upon release, the damper assembly falls on the string or string set. For even damping across the entire range of the piano, the weight of the damper assembly is varied in accordance with the size and tension of the string or string set so as to have a correspondingly even damping influence on the resonance thereof. The damper felt has a longitudinal aspect which for proper damping action needs to be in full contact with the string or string set. At a pre-determined node when a string set is not level, the damping mechanism does not seat properly thereon and results in a partially damped note. This is created by one or two strings ceasing to resonate and by the remaining undamped string sustaining the note until the natural decay occurs. 
   In manufacturing a grand piano the process of string leveling is accomplished prior to voicing which was described by Franz Mohr of Steinway as open work (or work with the piano action withdrawn from the piano case). The manufacturer anticipates that during the initial period of use, the instrument will be tuned, string leveled and voiced with greater frequency. This results as the outer portion of the hammer felts continue to be further shaped by the initial play by the pianist. Additionally, the tuning-string-leveling-voicing process takes on particular importance when a pianist individualizes the grand piano for concerts or recording sessions. When voicing is attempted without the pre-requisite string leveling, the shaping and the hardening of hammer felts do not provide the desired clarity as a hammer strike on an uneven string set produces tones with differing decays. 
   Similarly, in regulating the mechanical arrangement of the una corda pedal, unlevel strings create a condition in which the hammer strikes, instead of squarely meeting the string (bichord) or string set (trichord), are frequently glancing blows. This produces an uneven tonal quality that is readily heard because of the pianissimo mode. 
   The effect of unlevel string sets when adjusting the damper mechanism is described in a December, 2002 article in the  Piano Technicians Journal . Andrew Remillard in an article entitled  Dampers: Peace and Quiet at Last  indicates that for proper adjustment of the dampers the strings need to be level because, “if one string hangs a little beneath its neighbor it will be virtually impossible to ever completely dampen it.” 
   Prior to preparing this application, the inventor became familiar with several patents concerned with grand piano manufacture and maintenance, which patents are included herein as further background material. The patents are: 
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
                 
             
             
               ITEM NO. 
               U.S. PAT. NO. 
               INVENTOR 
               ISSUE DATE 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               1. 
               6,559,369 
               Gilmore 
               May 6, 2003 
             
             
               2. 
               6,489,548 
               Schindler 
               Dec. 3, 2002 
             
             
               3. 
               6,479,738 
               Gilmore 
               Nov. 12, 2002 
             
             
               4. 
               6,278,047 
               Cumberland 
               Aug. 21, 2001 
             
             
               5. 
               6,107,556 
               Gilliam 
               Aug. 22, 2000 
             
             
               6. 
               5,756,913 
               Gilmore 
               May 26, 1998 
             
             
               7. 
               5,654,515 
               Youse 
               Aug. 5, 1997 
             
             
               8. 
               5,528,970 
               Zacaroli 
               Jun. 25, 1996 
             
             
               9. 
               5,423,241 
               Scarton et al. 
               Jun. 13, 1995 
             
             
               10. 
               5,065,660 
               de Buda 
               Nov. 19, 1991 
             
             
               11. 
               4,253,374 
               Watterman 
               Mar. 3, 1981 
             
             
               12. 
               4,127,051 
               Conklin, Jr. 
               Nov. 28, 1978 
             
             
               13. 
               3,675,529 
               Van Der Woerd 
               Jul. 11, 1972 
             
             
                 
             
           
        
       
     
   
   Additional background information was obtained from several nonpatent references, namely:
     14. 40 TH  Annual PTG Convention (1997)—Mini Technical Classes—Michael Vecchione,  Making Unisons Sound SPOT ON      15. 40 TH  Annual PTG Convention (1997)—Mini Technical Classes—Clair Davies,  String Leveling      16. 1998 www.ptg.org—John Woodrow,  String Leveling Questions      17. Del Fandrick, RPT,  Q. and A. On the Level, Piano Technicians Journal  ( PTJ ), Vol 38, No. 2 (1995)   18. Clair Davies, RPT  String Leveling by Ear, PTJ . Vol. 40, No. 11 (1997)   19.  Solution for Agraffe Noise, PTJ , Vol. 32, No. 8 (1989)   20.  Q. and A. Fuzzy, False Tone, PTJ , Vol. 29, No. 3 (1986)   21. Virgil E. Smith  Criminal Negligence in Piano Service, PTJ , Vol. 36, No 10 (1993)   22. David W. Pitsch, RPT,  After Touch, PTJ , Vol 25, No 12 (1982)   23. Susan Graham,  Agraffes, PTJ , Vol. 27, No. 5 (1984)   24. Selections from  Five Lectures on The Acoustics of the Piano Proceedings of the Royal Swedish Academy of Piano  (1990) as published at www.speech.kth.se   

   In addition to the patents uncovered, the nonpatent references, as cited above, uncovered a series of five lectures entitled,  The Acoustics of the Piano , sponsored by the Royal Swedish Academy of Music (1990). While there was some emphasis on aftersound and the physics of sound upon a hammer striking a string, there is seemingly no mention of the effect of unlevel strings on the voicing or on the damping mechanism. In fact, the discussion seems to indicate that some roughness at the edge of the notes was deliberate, which teaches away from the precise string leveling, infra. 
   In Clair Davies teaching before the Piano Technicians Guild (PTG) convention about string leveling, he uses the square end of a steel rule to check that the strings are coplanar and does not use the key bed as reference. This teaching also indicates that Franz Mohr called string leveling part of the “open work” of piano tuning. 
   With the foregoing background in mind, the purpose of the present invention is to provide a novel string leveling device which precisely gauges the condition of the bichords and trichords of a grand piano with reference to the upper surface of the key bed. The disclosed invention also encompasses a method of using the apparatus as a preliminary step to voicing a grand piano, adjusting the damping mechanism and regulating the una corda pedal. 
   SUMMARY OF THE INVENTION 
   Briefly, in accordance with one aspect of the present invention, an apparatus or unison gauge for indicating levelness of a string set includes a contact block adjustably engageable with the lowermost tangential aspect of the string set under examination and a set of battery powered indicators providing an indication for each string. With the piano action withdrawn from the piano case, the base of the device is designed to rest on the key bed below the piano hammer strike area of the string set and to hold the contact block, which is mounted thereon on an extendable mast or probe, with the contact face thereof in a plane parallel to the key bed. The mast can be raised or lowered to conform the gauge to the grand piano being examined. Each contact block provides at least two contact points for each string of a string set and has a corresponding indicator therefor. When in the case of a trichord with all strings level, in this embodiment, the strings contact all six contact points and completes the circuits for all three indicators. The contact block is under slight spring tension when engaged with a string set, which is being monitored and, upon release of the spring tension readily slides along the key bed to the next string set. The unison gauge of this invention brings precision to string leveling, while at the same time saving several hours of the piano technician&#39;s time. 
   The present invention is effective in measuring the levelness of the string sets of a grand piano and is useful at the manufacturing level for quality assurance and is of even greater use in field maintenance procedures where string leveling is a preliminary step to voicing, damping mechanism adjustment, and una corde pedal regulation. The gauge is relatively small, portable, handheld, and is easy to use by a piano technician during grand piano production or field maintenance of grand pianos. 
   In method of use of the Russo unison gauge of this invention (as the hammer strike areas of the various unisons are arrayed on a slightly arced path following the bridge of the grand piano), the gauge is first adjusted so that the altitude upon extension is slightly greater than the highest unison with respect to the key bed surface. The gauge is then aligned so that the contacts for a particular string are in line. This assures that when a 3×2 contact array is elevated to contact, for example, a trichord string set, all six contacts are in contact upon finding a level string set. If an indication of an unlevel condition occurs, the sagging string is then shortened or lifted to the right height using a string hook (or a low-effort string hook—see Reference 18 by Clair Davies, supra. 
   OBJECTS AND FEATURES OF THE INVENTION 
   Accordingly, it is the primary object of the present invention to provide a new and novel device to ascertain the levelness of string sets of a grand piano. 
   It is another object of the present invention to provide a new and novel device to ascertain that the hammer strike points of a string set—namely a bichord or a trichord—lie in a line parallel to the key bed. 
   It is yet another object of the present invention to provide a levelness device for a grand piano which is easy to use and economical to manufacture. 
   It is still yet another object of the present invention to provide a device that eliminates piano aftersound by precisely leveling string sets. 
   It is a feature of the present invention that the string leveling device hereof is readily moved from one string set to another and has a baseplate for using the upper surface of the key bed as a reference plane. 
   It is another feature of the present invention that the contact surface of the string leveling device is readily raised into and lowered from tangential engagement with the string set at the hammer strike point. 
   It is yet another feature of the present invention that the extension of the string leveling device encompasses the range of elevation of the strings sets in presently marketed grand pianos. 
   Other objects and features of the invention will become apparent upon review of the drawing and the detailed description which follows. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawing as hereinafter described, preferred embodiments are depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention. Also, in the drawing which follows, the same part in the various views is provided the same reference designator. 
       FIGS. 1 and 1A  are partial perspective views of a grand piano with the piano action withdrawn and the unison gauge of this invention positioned between the piano key bed and a trichord with  FIG. 1  showing the details of the lyre structure, and  FIG. 1A , of the damper mechanism; 
       FIG. 2  is a partially cutaway perspective view of the unison gauge of  FIG. 1  showing the mast elevation mechanism for adjusting the gauge to the piano under string leveling; 
       FIG. 3  is a cross-sectional view of the unison gauge showing the mast extended to bring the contacts of the contact block into tangential engagement with a level trichord having a sagging string; 
       FIG. 4  is a cross-sectional view of the unison gauge showing the mast extended to bring the contacts of the contact block into tangential engagement with the trichord being leveled; 
       FIG. 5  is a detailed view providing a schematic representation of the indicator panel, the circuitry and power supply arrangement; 
       FIG. 6  is a detailed view of a interchangeable contact block for the unison gauge of  FIG. 2  for tangential engagement with a bichord unison; 
       FIG. 7  is a partially cutaway perspective view of a second embodiment of the unison gauge of this invention using a series of precision base blocks to elevate the device; 
       FIG. 8  is a partially cutaway perspective view of a third embodiment of the unison gauge of this invention, which embodiment accommodates the gauging of both bichords and trichords; and, 
       FIG. 9  is a view of a piano technician shortening a sagging string of a string set to bring all the strings in the unison to a level condition. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Before embarking on the description of the device, several terms are defined for the purpose of adding clarity to this specification and the claims which follow. Here the term unison refers to a string set, which, in turn, is a bichord or trichord, that is played as one note by the percussive force of a piano hammer. The term bichord is defined as a unison or string set having two strings that is played as one note by the percussive force of a piano hammer. The term trichord is defined as a unison or string set having three strings that is played as one note by the percussive force of a piano hammer. The term voicing is defined as the final adjustment of a piano and includes trimming, hardening, needling, and, if necessary, replacing the hammer felts. The term damping mechanism adjustment is defined as the procedure by which the dampers and the damper lift mechanism are adjusted to ensure accurate damping performance. The una corda pedal is defined as the pedal for shifting the piano action so that, while the pedal is depressed, the respective piano hammers strike only one string of a bichord and only two strings of a trichord. 
   Referring now to  FIGS. 1 and 1A  partial perspective views of a grand piano are shown with the piano action withdrawn and the unison gauge of this invention positioned between the piano key bed and a trichord. The Russo unison gauge is referred to generally by the reference designator  20 . A grand piano  22 , with which the unison gauge  20  of this invention is used, is shown as having a piano case  24  with a piano action  26  withdrawn therefrom. Surrounding the piano action  26  is the piano case  24  and beneath the withdrawn piano action  26  and extending rearward therefrom is a key bed  28 —a flat expanse providing support for the piano action  26 . Depending from the piano case  24  is a pedal-supporting lyre structure  30  with a una corda pedal  32  on the left side thereof. Across and atop a string assembly  34 , a damper mechanism  36  is disposed with a damper  38  to cut off the resonating of each associated string or string set  40 . In this figure, the unison gauge  20  is shown extended between the key bed  28  and the string set  40  of string assembly  34 . 
   Referring now to  FIGS. 2 and 3  are respectively perspective and sectional views of the unison gauge of the invention shown in  FIG. 1 . Here the mast elevation mechanism for adjusting the external gauge range to the piano under string leveling. Each unison gauge  20  has two operating ranges, namely, (1) an external range  42  and (2) an internal range  44 . The external range  42  of a unison gauge  20  for a grand piano depends on and is fixed by the manufacturer, and as there is no industry-wide standardization, the maximum height between key bed and string set varies brand-to-brand from 6.5 to 9.5 inches. The string array  46  of string assembly  34  is slightly arcuate and in a grand piano wherein the external range is for example 7.75 inches (max.) the minimum-to-maximum range of key bed to string set heights is from 7.00 inches to 7.75 inches. This then determines the internal range  44  of operation of a unison gauge. 
   Returning to  FIG. 2 , the structure of the unison gauge  20  is next discussed. In this embodiment, the gauge  20  has a flat base  48  for emplacement of the gauge on key bed  28 . As will be ascertained from the discussion, infra, the key bed  28  is used as a reference plane and the flatness characteristic of the base  48  is translated into the precision gauging of the string set levelness. Upon the base  48 , a gauge housing  50  is constructed and receives therein a mast assembly  52  which is slidably mounted to move along an axis line  54  normal to the lower surface  56  of base  48  and the upper surface  58  of key bed  28 . The mast assembly  52  is a gear arrangement that raises and lowers the entire mast assembly  52  to a fixed position thereby setting the external range  42 . Here, the gear arrangement consists of worm or drive gear  60  and mating or driven gear  62  with the drive gear  60  rotated by control knob  64 . While shown herein by a worm gear unit, any suitable mechanical device that translates the motion into displacement along the mast axis including, but not limited to a slider-crank mechanism, a ratchet, or a rack and pinion arrangement, would be within the contemplation of this invention. 
   Referring now to both  FIGS. 2 and 3 , the mast assembly  52  is described in some detail. The mast  66  includes a support mast or fixed portion  68  and a slidable mast or movable portion  70  for extension and retraction along channel  72  of support mast  68 . Although the support mast  68  is shown as female and the slidable mast  70  is shown as male, the interengagement function is readily reversible. Likewise, the slidable mast  70  is optionally constructed as telescoping within the support mast  68 . A spring  74  extends from a lower spring holder  76  attached to base  48  to upper spring holder  78  attached to slidable mast  68 . When the spring  74  is relaxed (not compressed), the unison gauge  20  is in the fully extended condition—limited, of course, by the selected external range  42 . 
   Medial the slidable mast portion  70 , a handle  80  is attached thereto and adapted to retracting or lowering the mast assembly  52  by compressing and holding compressed spring  74 . Compression of the spring in this manner releases the gauge that was held between the key bed and a particular unison and enables the operator to move the gauge to the next unison to be checked for levelness. 
   Referring now to  FIG. 3  a cross-sectional view is shown of the unison gauge  20  with the mast extended to bring the contacts of the contact block into tangential engagement with a level trichord. At the end of the slidable mast portion  70  opposite the base  48 , a contact block  82  is mounted thereon. The contact block  82  is constructed with the upper surface  84  thereof lying in a plane parallel to lower surface of base  48 . The upper surface  84  of contact block  82  contains contact points  86  arrayed in grid which adapts the structure for tangential engagement with the trichord being leveled. The contact block  82  has two contact points  86  for each string and, when the trichord is level, each string is in contact with both of the corresponding contact points  86 . With the geometry of the gauge as discussed above, indication that each string of the specific trichord is level shows: (1) the two contact points lie in a line  88  parallel to lower surface  56  of base  48  and upper surface  58  of key bed  28 ; (2) the six tangentially engaged contact points lie within three parallel lines  88  in upper surface  84  of contact block  82 ; and, (3) because of the internal range  44 , while the plane of each trichord may be at a slightly different altitude with respect to upper surface  58  of key bed  28 , the plane is parallel to the key bed. 
   Referring now to  FIG. 4  a cross-sectional view is shown of the unison gauge  20  with the mast extended to bring the contacts of the contact block into tangential engagement with a trichord having a sagging string. It is noted that the spring tension of spring  74  at the relaxed condition is selected so as to be insufficient to impel a sagging string of a unison to the properly aligned position. Thus, a sagging string will always create a non-indication of levelness with respect to the other strings in the string set and will not provide a level indication until the condition is remedied. 
   The unison gauge  20  provides indication of levelness through a series of indicators. The indicators shown are battery-powered, light emitting diodes mounted on gauge housing  50 . The physical positioning of the indicators is best seen in  FIGS. 2 ,  3  and  4  with the circuitry in  FIG. 5 . A battery compartment access panel  90  slides to one side and covers a 1.5 volt battery  92  adjacent the base  48  of unison gauge  20 . For the trichord unison  40 , a light emitting diode (LED)  94  is associated with each string of the string set. A wiring harness  96  extends from the contact block  82  mounted on the slidable mast  70  to an indicator panel  98  on gauge housing  50 . Three parallel lines accommodate the LED&#39;s  94  with the circuit for each indicator being completed by the respective string. A low battery indication is optionally provided. 
   Referring now to  FIG. 6 , an interchangeable contact block  100  is shown. This contact block is used for determining the string levelness of a bichord unison  102 . In lieu of the 3×2 matrix of contact block  82 , here a 2×2 matrix of contact points  104  is employed. For indication of levelness a two-branch parallel circuit structure  106  is associated with LED&#39;s  108 . 
   A second embodiment of the unison gauge of this invention is shown in  FIG. 7 . Here similar parts to those in the first embodiment are provided with reference designators  100  units higher than those in the first embodiment. Thus the mast assembly  152  of the second embodiment is similar to mast assembly  52  of the first embodiment. In this embodiment the means of elevating the unison gauge  120  differs from that described, supra. In lieu of an internal mechanism, precision-machined base blocks  161  and  163  that nest the one with the other and with the base  148  are used to elevate the unison gauge  120 . While these base blocks increase the external range  142  by 1-inch increments from 6.4 inches to 9.5 inches, special base blocks are optionally available for given makes of grand pianos, e.g. a Steinway block or a Kawai block. 
   A third embodiment of the unison gauge of this invention is shown in  FIG. 8 . Here similar parts to those in the first embodiment are provided with reference designators  200  units higher than those in the first embodiment. Thus the mast assembly  252  of the third embodiment is similar to mast assembly  52  of the first embodiment. In this embodiment the unison gauge  220  incorporates both a trichord mode and a bichord mode and indicates levelness in either mode. A mode switch  293  switches between a three-light set of light emitting diodes  294  with an associated three-branch, parallel circuit, as above, and a two-light set of light emitting diodes  295  with an associated two branch, parallel circuit. The contact block  282  uses the 3×2 matrix  304  thereof to ascertain the levelness of trichord unisons and upon switching modes the same (now 2×3) matrix  304  to ascertain the levelness of the bichord unisons. The technician using the gauge needs only to switch the mode and to rotate the gauge  220  at a right angle to the other mode. 
   In operation, string leveling is performed as a preliminary step to several manufacturing and maintenance procedures for the grand piano. As indicated above, these include voicing, damping mechanism adjustment and una corda pedal action. The rationale for leveling strings is slightly different in each case but significantly each string leveling operation concerns the interaction of felted, movable mechanical parts with piano strings. When a damper head falls on a unison, if two strings are high and one is low, the low string will not be properly damped. When a hammer strikes an unlevel string set, a complex of tones, rather than a single one, emanates from the unison. When a una corda pedal is depressed, an unlevel string, which is purposely rendered inoperative by the pedal action, often catches the corner of a piano hammer and does not provide the desired pianissimo effect. String leveling is considered preliminary because the strings provide a “roadway” on which the felted, movable mechanical parts operate and it is more sensible to fix the “potholes” than all the suspension problems arising from a rough ride. 
   A note is now inserted about selection of the external range. The selectability of the external range is only important to the Russo unison gauge user who maintains or re-manufactures a variety of grand pianos. This invention contemplates the ability to construct unison gauges operable over the entire external range without adjustment and also unison gauges which have fixed external ranges. For these reasons the selection is considered an optional step in the methodology. 
   The general steps of string leveling are as follows: 
   a. withdrawing the piano action from the piano case to provide access to the key bed and the strike area of the string sets; 
   b. optionally selecting the external range for the unison gauge, with the compression capability thereof including the minimum and maximum altitudes of the string sets being leveled; 
   c. placing the unison gauge on the key bed having the contact points of the gauge head aligned with the longitudinal axis of the strings of the string set and having a row of contact points for each string of a string set; 
   d. compressing the unison gauge by using the handle therefor and moving the gauge head downwards towards the base; 
   e. releasing the gauge for tangential contact between the string-set-under-test and the contact points of the gauge head; 
   f. completing the circuit for the LED indicators and observing the display; 
   g. shortening, as required, the sagging string of the string set, see  FIG. 9 ; 
   h. repeating steps c. through f. until all contact points are indicated as being in the same plane; 
   i. compressing the unison gauge by using the handle therefor and moving the gauge to the next string-set-under-test; and, 
   j. repeating steps e. through h. until all unisons have been tested for levelness. 
   The following steps of string leveling applies to the embodiment of the unison gauge which uses two separate contact heads, namely, one for trichord and the other for bichords: 
   a. through i. using the trichord contact head repeat steps a. through h. as in the preceding paragraph; 
   j. (not used); 
   k. repeating steps e. through h. until all trichords are indicated as being level; 
   l. demounting the trichord contact head and securing in lieu thereof the bichord contact head; and 
   m. repeating step e. through i. until all bichords are indicated as being level. 
   While the preferred embodiment of our invention has been described fully in order to explain its principles, it is understood that various modifications or alterations may be made to the preferred embodiment without departing from the scope of the invention as set forth in the appended claims.