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quarter-tone. An interval of half a semitone (24 quarter-tones to the octave). Introduced into Western mus. in 20th cent. with rise of interest in other cultures, quarter-tones having always been used in Eastern mus. Poses special problems of notation. Some composers who have written in quarter-tones have built special pfs., e.g. Hans Barth and Alois Hába. Boulez, Stockhausen, and other avant-garde serial composers have used quarter-tones.