quarter tone

In music, an interval half that of a semitone. A kind of microtone, it is the smallest conventional interval, commonly used in Western music by 20th-century composers such as Pierre Boulez and Karlheinz Stockhausen.

Quarter tones were known to the Greeks and were apparently used in early plainsong but soon came to be abandoned and were not revived in Eastern music until the 20th century. The chief exponent of quarter-tone music is Alois Hába; others, such as Béla Bartók and Ernest Bloch, have used the device, but not systematically.