Regulation is a connected series of procedures that align the parts of a piano action. Proper regulation of keys to action, and action to strings ensures that a piano is playing most efficiently. Pianos are regulated first in the factory after assembly, and can require additional regulation after being shipped to a dealer. Piano keys and actions were designed to be adjusted, as the parts are affected by wear and changes in humidity, and will change from their proper adjustments.
Once in the home, the regulation of a piano is dependent on the amount of use that it receives. Constant playing will settle the leveling of the keys, changing the responsiveness of the action to the keys, and creating a different feel or touch to the player. A piano in poor regulation is hard to play, and the tone (see voicing) is also affected.
Regulation, like tuning , is a normal, ongoing service concern, and should be addressed with each tuning a piano receives. It is far simpler (and cheaper!) to correct small changes in regulation at each tuning than to perform a major regulation when the piano becomes difficult to play.