Jazz

One of the largest stated influences on the sound of The Birth of the Cool was band leader Claude Thornhill and his orchestra. Out of Thornhill's band came Lee Konitz, Gerry Mulligan, and Gil Evans, Miles Davis calling the Konitz-Mulligan-Evans incarnation "the greatest band" only after "the Billy Eckstine band with Bird." The Thornhill band was known for its impressionistic style, innovative use of instrumentation, such as the use of tuba and French horn, and a non-vibrato playing style, hallmarks that the Miles Davis Nonet adopted for The Birth of the Cool. According to Evans:

Miles had liked some of what Gerry and I had written for Claude. The instrumentation for the Miles session was caused by the fact that this was the smallest number of instruments that could get the sound and still express all the harmonies the Thornhill band used. Miles wanted to play his idiom with that kind of sound.

Davis saw the full 18-piece Thornhill orchestra as cumbersome and thus decided to split the group in half for his desired sound. As arrangers, both Evans and Mulligan gave Thornhill credit for crafting their sound. Thornhill's band gave Evans the opportunity to try his hand at arranging small-group bebop tunes for big band, a practice few others were participating in. Mulligan recalls Thornhill teaching him "the greatest lesson in dynamics, the art of underblowing." Thornhill has also been credited with launching the move away from call and response between sections and the move towards unison harmonies.