(1) The quality of a musical passage or composition that causes it to be sensed as gravitating towards a particular note, called the key note or the tonic. One therefore speaks of a piece as being in C major or minor etc. See Tonality.

Key is also taken to mean pitch; the expression to sing ‘off key’ signifies out of tune; ‘on key’ can mean at concert (i.e. standard) pitch.

(2) In keyboard instruments, a balanced lever which when depressed either operates a valve to admit air to a pipe or reed or mechanically strikes or plucks a string. In mouth-blown instruments, the key is a device controlling a tone hole out of the reach of, or too large for, the fingers: it consists of a touchpiece for the finger, a pivoted lever and a padded plate or cup to close the hole.