A quarter tone is a musical interval equal to exactly half a semitone. Semitones are 100 cents, so quarter tones are 50 cents and 24 of them make an octave. (Thus a quarter tone is the twelfth root of two or approximately 35/36.) "quarter tone" is also the name given to any pitch halfway between two adjacent conventional pitches, for instance between F and F#. Quarter tones appear in the enharmonic genus of ancient Greek music and also in Arabic maqamat. The earliest known published Western quarter-tone composition is Richard Stein's Zwei Konzertstück (1906). It is possible to play quarter tones on any fretless stringed instrument, an instrument with a slide such as a trombone, or an instrument modified to play quarter tones either by adding more keys to a keyboard or by adding a special quarter tone valve to a valved brass instrument. It is possible, though difficult, to sing quarter tones. I avoid quarter tones myself because for one thing they don't sound good, and for another they're just another form of equal temperament with 24 equal divisions instead of 12.