Microtonal music is music using microtones — intervals of less than an equally space semitone. Microtonality / microtonal music can also refer to music which uses intervals not found in the Western system of 12 equal intervals to the octave. Microtonal music may refer to all music which contains intervals smaller than the conventional contemporary Western semitone. The term implies music containing very small intervals but can include any tuning that differs from the western 12 tone equal temperament. By this definition, the following systems are microtonal: a diatonic scale in any meantone tuning; the traditional Carnatic system of 22 sruti; much Indonesian gamelan music; and Thai, Burmese, and African music which use 7 tones in each (approximate) octave. Hence, the term "microtonal" is used to describe music using intervals not found in 12-tone equal temperament, so these musics, as well as musics using just intonation, meantone temperament, or other alternative tunings may be considered microtonal.
Other terminology has been used (and is still used today) by theorists and composers. Micro-intervals is commonly used to speak about intervals smaller than the semitone, and sometimes macro-intervals for non-multiples of the semitone greater than it. Ivan Wyschnegradsky (and many of those inspired by him) used the term ultra-chromatic for micro-intervals and infra-chromatic for macro-intervals (Wyschnegradsky 1972, 84-87). Ivor Darreg proposed the term xenharmonic (from the Greek ξ?νος, foreign, and Greek ξεν?α, hospitable) for any scale other than 12-tone equal tempered scale. (See xenharmonic music).
One reason microtonalists explore alternate tunings is that each unique even or uneven division of the octave or non-octave or octave+fifth etc. gives a new world of intervallic connections and thereby new musical content. Just-intonation scales like Partch's 43 tone unequal scale start with the (non-tempered) diatonic Western scale, and many of them extend it, in Partch's case up to the 11th partial (Partch 1979, 93, 119-137). Some like the 19 tone or 31 tone equal scales may be used close to diatonic scales, but extend them considerably. Other divisions of the octave do not support the diatonic basis for Western musical notation and tonal theory, but have other equally viable intervallic relationships.
While experimenting with his violin in 1895, Julian Carrillo (1875-1965) discovered the sixteenths of tone, i.e., sixteen clearly different sounds between the pitches of G and A emitted by the fourth violin string. He named his discovery Sonido 13 (the thirteenth sound) and wrote on music theory and the physics of music. He invented a simple numerical musical notation that can represent scales based on any division of the octave, like thirds, fourths, quarters, fifths, sixths, sevenths, and so on (even if Carrillo wrote, most of the time, for quarters, eights, and sixteenths combined, the notation is able to represent any imaginable subdivision). He invented new musical instruments, and adapted others to produce microintervals. He composed a large amount of microtonal music and recorded about 30 of his compositions.
Major microtonal composers of the 1920s and 1930s include Alois Hába (quarter tones, or 24 equal pitches per octave, and sixth tones), Julian Carillo (24 equal, 36, 48, 60, 72, and 96 equal pitches to the octave embodied in a series of specially custom-built pianos), Ivan Wyschnegradsky (third tones, quarter tones, sixth tones and twelfth tones, non octaving scales) and the early works of Harry Partch (just intonation using frequencies at ratios of prime integers 3, 5, 7, and 11, their powers, and products of those numbers, from a central frequency of G-196) (Partch 1979, chapt. 8, "Application of the 11 Limit", 119–37).
Prominent microtonal composers or researchers of the 1940s and 1950s include Adriaan Daniël Fokker (31 equal tones per octave), Partch again (continuing to build his handcrafted orchestra of microtonal just intonation instruments) and Ivor Darreg (who built the first fully retunable electronic synthesizer capable of any division of the octave, just or equal or non-just non-equal).
Prominent microtonal composers of the 1960s and 1970s include John Eaton (who created his own microtonal synthesizer, the Syn Ket, to produce microtonal intervals), Ivor Darreg again (who augmented his home-built orchestra of instruments to include guitars refretted in equal temperaments 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 24, and 31, as well as the magalyra series of sub-contrabass steel guitar instruments), Harry Partch, Easley Blackwood (who composed and performed the well-known Twelve Microtonal Etudes for Electronic Music Media with compositions in every equal division of the octave from 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23 and 24 equal pitches per octave) and Augusto Novaro, the Mexican microtonal theorist who composed studies in 15 equal, among others. Barbara Benary also formed Gamelan Son of Lion around this period, and Lou Harrison was instrumental in creating American gamelan orchestras at Mills College. In Europe, the "Spectralists" in Paris created their first works from 1973 on with an extensive use of microtonal harmony. The main composers were Hugues Dufourt, Gérard Grisey, Tristan Murail and Michael Levinas; see also the parisian ensemble "L'itinéraire". György Ligeti in Hamburg strongly promoted microtonality and used it in several of his works.