Regulation
Over time, the performance of a piano action tends to decline, due to the compression of felt, warping of wood, and other types of wear.
A skilled technician can restore it to optimal precision, in a process called regulation, which involves adjustments ranging from turning a small screw to sanding down a wood surface.
Many new pianos are not perfectly regulated when released from the factory, or quickly lose their regulation when moved to their new home.
The goal of regulation is to make the piano's touch and sound consistent across all notes, allow it to comfortably achieve the widest possible range of dynamics, and make the keys responsive to even the most rapid or most subtle motions of the player.