Microtone

Microtonal music is music using microtones — intervals of less than an equally spaced semitone, or as Charles Ives put it, the "notes between the cracks" of the piano. A looser definition includes anything not in 12-tone equal temperament, while a stricter definition distinguishes between microtonal music (which can take 12-tone equal temperament as an audible basis), and xenharmonic music (which does not).
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