key

1. On a piano, organ, harpsichord, virginal, etc., the keys are the levers which the performer depresses to activate the action, thus producing tones.

 

2. On a woodwind instrument, the keys are the metal disks that close or open soundholes by means of levers operated by the performer's fingers. Keys are used for those soundholes that are uncomfortable or impossible for the performer to reach with his/her fingers. 

 

3. A specific scale or series of notes defining a particular tonality. Keys may be defined as major or minor, and are named after their tonic or keynote. Thus the series of notes with intervals defining a major tonality and based on the key of C is the key of C major.