1 As a principle in mus. comp., implies adherence, in any passage, to the note‐material of one of the major or minor scales (see scale)—not necessarily a rigid adherence (since other notes may incidentally appear), but a general adherence, with a recognition of the tonic (or key‐note) of the scale in question as a principal and governing factor in its effect. For instance we speak of a passage as being ‘in the key of’ C major, or F minor, and also use the same terms to describe a comp. (or movement) as a whole—in this latter case implying merely that the key mentioned is that in which the piece begins and sometimes but not always (e.g. Mahler) ends and is its governing one (see modulation). If a piece in several movements is so spoken of it does not necessarily mean more than that the first movement (usually also the last one) is in that key.
The element of key crept into European mus. in the early 17th cent., as the modes gradually fell out of use: it remained of supreme importance to the end of the 19th cent. but in the 20th cent., many composers, led by Schoenberg, have abandoned tonality. See atonal.
2 A lever on an instr. which is depressed by finger or foot to produce a note, e.g. on a pf. by finger, on an org. by foot, on woodwind by finger (the levers covering the airholes).