From wikipedia:
Holton built Ellis a custom trumpet which he received in September 1965.[15] Its additional (fourth) valve enabled it to produce quarter tones. The inspiration for this may have been due to his studies of Indian music, which includes bent pitches that some ethnomusicologists refer to as “microtones”. However, it was probably more the result of Ellis’s involvement with avant-garde classical music in which many composers were experimenting with Western tonality and intervals, especially Harry Partch, with whom Ellis is known to have met and discussed ideas.
https://en.wikipedia.org/wiki/Don_Ellis
Microtones refer to intervals smaller than a semitone, the smallest interval used in Western music. While the Western musical tradition typically divides the octave into 12 equal parts (semitones), other musical cultures and composers have explored smaller intervals, known as microtones or “microtonal” music.
Don Ellis, an American jazz trumpeter, composer, and bandleader, was known for his innovative approach to music, which often incorporated elements of avant-garde and experimental music. Ellis was particularly interested in exploring unconventional scales and tunings, including microtonal systems.
One of Ellis’s notable contributions to microtonal music was his use of quarter-tone scales, where the octave is divided into 24 equal parts instead of 12. This allowed for a greater range of pitches and intervals, expanding the harmonic and melodic possibilities of his compositions.
Ellis’s interest in microtonal music was evident in his compositions, arrangements, and performances, where he incorporated quarter-tone scales and other microtonal techniques to create unique and unconventional sounds. His experimentation with microtones contributed to the development of avant-garde jazz and influenced subsequent generations of musicians and composers exploring alternative tuning systems.
Overall, Don Ellis’s exploration of microtonal music reflects his pioneering spirit and willingness to push the boundaries of traditional musical conventions. His contributions to microtonal music have left a lasting impact on the world of jazz and experimental music, inspiring artists to explore new sonic territories and expand the possibilities of musical expression.