24 equal temperament

A quarter tone  play , is a pitch halfway between the usual notes of a chromatic scale or an interval about half as wide (aurally, or logarithmically) as a semitone, which is half a whole tone.

Many composers are known for having written music including quarter tones or the quarter-tone scale (24 equal temperament)—proposed by 19th-century music theorists Heinrich Richter in 1823 and Mikha'il Mishaqah c. 1840—including: Pierre Boulez, Julián Carrillo, Mildred Couper, George Enescu, Alberto Ginastera, Gérard Grisey, Alois Hába, Ljubica Marić, Charles Ives, Tristan Murail, Krzysztof Penderecki, Giacinto Scelsi, Ammar El Sherei, , Tui St. George Tucker, Ivan Alexandrovich Wyschnegradsky, and Iannis Xenakis (see List of quarter tone pieces).

...
Wikipedia