A piano’s action is an assemblage of levers that move in intersecting arcs of motion, and these parts must be installed in correct relationship to each other in order to operate optimally. These relationships are known as the action’s geometry. Even in fine pianos, many reasons for geometry problems can arise. Some new pianos leave the factory with these problems, but most often they result from rebuilds done over the last 40 years, a period during which replacement parts in the original dimensions were often unavailable. In some cases, a piano born with poor action geometry has been rebuilt, and the problems have become even worse.
In the last 15 years, better parts have become available, and action manufacturers are now much more astute in anticipating the problems that can arise in replacing action parts. Piano technicians, too, have developed more sophisticated methods of addressing geometry issues, including: changing the locations of specific action parts, resetting the position of the entire action stack relative to the keyboard and strings, adding or removing action cloth, rehanging hammers, or installing a completely new action. Most important is that the proper analysis be made of the extent to which poor action geometry is contributing to the problems reported by the player.
As keys are played, pads of cloth, leather, and other materials serve to silence the action parts as they move. As these materials pack down and wear with use, the positions of the parts change slightly over time, and periodically need to be brought back to their original positions in order for the instrument to play properly, a procedure known as action regulation.