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Cool jazz is a jazz style that emerged in the late 1940s in New York City.

History

During 1946, after the Second World War, there was an influx of Californian (predominantly white) jazz musicians to New York. Once there, these musicians mixed with the mostly black bebop musicians, but by the "light" sound of Lester Young (Pres) in particular. The style that emerged became known as "cool jazz", which avoided the aggressive tempos and harmonic abstraction of bebop.
Cool jazz had several sources and tributaries. Arrangers Gil Evans and Gerry Mulligan developed their initial ideas working for the Claude Thornhill Orchestra, which featured such then-unheard-of instruments (for jazz) as french horn and tuba; the added forces permitted Evans and Mulligan to explore softer emotional and timbral shading than had been typical of swing-era big bands. Another variety of "cool jazz" was that of the pianist Lennie Tristano and his students, notably the saxophonists Lee Konitz (who spent some time in the Thornhill band) and Warne Marsh. Tristano's music is very different from what Evans and his colleagues were up to: its "coolness" was a matter of emotional temperature (Tristano required saxophonists to play with a "pure" tone and to concentrate on melodic development and interaction rather than overt emotionalism), but his emphasis on sometimes ferociously fast tempos and on pure improvisation rather than arrangement was closer to bebop.
The classic confluence of these various streams came with the 1949-1950 sessions now best known under their later title: Miles Davis's Birth of the Cool (1957). Despite Davis's top billing, this was in fact a collective project that drew together many players and arrangers/composers from the period: Davis, Evans, Mulligan, Konitz, John Lewis, Gunther Schuller, and Johnny Carisi. Issued only shortly after bebop had begun to establish itself, it offered an alternative aesthetic that was initially unpopular – the recordings originally sold poorly and the band did not last long – but slowly established itself as a jazz classic.
Despite its impact in the New York scene, cool jazz later became strongly identified with the West Coast jazz scene. Its influence stretches into such later developments as bossa nova, modal jazz (especially in the form of Davis's Kind of Blue 1959), and even free jazz (in the form of Jimmy Giuffre's 1961-1962 trio).
Source
http://en.wikipedia.org/