The action mechanism of a key consists of the key itself and all its appurtenances. A key appurtenances include a see-saw like leverage-system, the escapement, a supplementary device for Repetition, and a check for hammer rebound.
The piano action was the key new addition that Bartolomeo Cristofori created when he invented the piano in 1700. Other than in the action, the first pianos were quite similar in construction to contemporary harpsichords. With the piano, a hammer strikes the string, whereas with a harpsichord, a mechanism plucks the string. Cristofori's action already embodied many of the principles still found in 2000-era actions. It used levers to magnify the small motion of the piano key into a large motion of the hammer, and was arranged so that the very last part of the hammer's motion before striking the string would be purely the result of inertia and not propelled by the key; this prevents the key from pressing the felt-covered hammer firmly into the string, which would damp and stop the string vibrations and the sound.
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