When a key on an acoustic piano is pressed down, the resulting sound continues for a considerable time after the press of the key has taken place, and stops when the key is released and allowed to rise. This elimination of sound is caused by action of a second piece of piano mechanism, called the damper. The dampers are small pieces of wood with felt attached to them.

This mechanism is so connected with the key that the latter cannot be moved without occasioning a movement of its own damper. Each key is thus the means by which both a hammer and a damper are moved to action, the hammer for producing tone, the damper for stopping it. (In the case of a few of the top keys of the piano, the damper mechanism is wanting.)

The damper lies constantly touching the string, except during the holding down of the key, when it is removed from its place and kept off the string as long as the key is held down. After a pushdown of the key, if the key is kept down, it will be noticed that (1) the hammer resting at its half-position and (2) the damper removed from the string. When the key is allowed to rise, the hammer falls completely back, and the damper returns to the string, and, by touching it, stops the tone.

If there were no provision for the stoppage of tone, the effect of any performance would be the same as that produced when the right foot pedal is held down during playing. The putting down of the pedal causing the removal of all the dampers from the strings, and creating in consequence the effect as of each separate sound floating about among all the others.

In the case of the hammer, it was pointed out that its complete work is finished in the shortest possible time, that it is finished instantaneously, and that although the key be kept down (and therefore may be understood to be fulfilling some function), still nothing more must be expected from the hammer.

It will be remembered that the key has two mechanisms depending upon it, namely, that for producing the tone, and that for stopping it. As long as the key is held down, it is doing a part of its work, as soon as the key is allowed to rise a change takes place and the tone stops.

It has been already shown that the work of the hammer is completed at the moment of the stroke. It must therefore be the work of the damper which the keeping down of the key is instrumental in furthering. As opposed to the hammer's work being finished as soon as the key is down, the work of the damper is not completed until the key rises. The work of the damper is to stop the tone; and as that stoppage cannot take place until the string is re-touched by the damper, which touch cannot take place until the key comes up; the work of the damper is therefore not finished until the key is allowed to rise.

The positive action of the hammer takes place when the key is pushed down. The positive action of the damper takes place when the key rises. The negative action of both hammer and damper, namely, their leaving the string, gives the latter freedom to vibrate after the stroke. While therefore both the hammer and the damper begin their work at the same moment, they complete it at different times, the hammer instantaneously, and the damper not until the piano key is allowed to rise.

All work done between the completion of the hammer's work and the completion of the damper's work is done by the sounding board of the piano; but as this intermediate work is altogether beyond control of the finger, it cannot come under any consideration concerning the manner of touching the keys.