The thing that separated the piano from prior keyboard instruments, such as the harpsichord or clavichord, was the ability of the piano to provide dynamics -- that is, the ability for the pianist to play loud, medium or soft.
Every piano has a mechanical device which translates the pressing down of a key to the sounding of a note. This device is referred to as the action. A piano action can be removed from the piano for servicing.
An upright action on the workbench awaiting service
A grand action outside of the piano
The action contains thousands of parts and adjustment to within certain tolerances is required in order for the piano to play properly. Each note has several different adjustment points that can be set so that all the keys play consistently with each other. This process of adjustment is referred to as action regulation. Depending upon the age of the piano, a certain level of preparation and repairs are usually required before the regulating process can begin. Interested in more? Continue with the action parts diagrams which also include animated demonstrations of each type of action.
Grand action parts
Upright action parts