Cool Jazz is a style of modern jazz music that arose following the Second World War.

Broadly, cool jazz refers to a number of post-war jazz styles employing a more subdued approach than that found in other modern jazz idioms.^[1] As Paul Tanner, Maurice Gerow, and David Megill suggest, "the tonal sonorities of these conservative players could be compared to pastel colors, while the solos of [Dizzy] Gillespie and his followers could be compared to fiery red colors."^[2]

Mark C. Gridley, writing for All Music Guide to Jazz, identifies four sub-categories, with considerable overlap, that encompass cool jazz:

1. "Soft variants of bebop", including the Miles Davis recordings that constitute Birth of the Cool; the complete works of the Modern Jazz Quartet; the output of Gerry Mulligan, especially his work with Chet Baker and Bob Brookmeyer; the music of Stan Kenton's sidemen during the late 1940s through the 1950s; and the works of George Shearing and Stan Getz.^[1]
2. The output of modern players who eschewed bebop in favor of advanced swing era developments, including musicians such as Lennie Tristano, Lee Konitz, and Warne Marsh; Dave Brubeck and Paul Desmond; and performers such as Jimmy Giuffre and Dave Pell who furthered Count Basie's small group music with Lester Young.^[1]
3. Musicians from either of the previous categories who were active in California from the 1940s through the 1960s, developing what came to be known as "West Coast jazz".^[1]
4. "Exploratory music with a subdued effect by Teddy Charles, Chico Hamilton, John LaPorta, and their colleagues during the 1950s.^[1]

There was an influx of Californian (predominantly white) jazz musicians to New York. These musicians mixed with the (predominantly black) bebop musicians already there and were influenced by the "smooth" sound of saxophonist Lester Young. The style that emerged became known as Cool Jazz because it avoided the aggressive tempos of bebop.

Cool Jazz included intricate arrangements, innovative forms, and songs having a thoroughly composed sound, although they did include improvised sections. The term "cool" started being applied to this music about 1953 with the release of the album Classics in Jazz: Cool and Quiet by Capitol Records.^[3]

Cool Jazz had several sources. Arrangers Gil Evans and Gerry Mulligan developed their initial ideas, working for the Claude Thornhill Orchestra featuring such then-unheard-of instruments, for jazz, as french horn and tuba.^[4]: 94 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 doing: its "coolness" was a matter of emotion (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 mixture of these various influences was during the 1949-1950 sessions now best known under their later title: Miles Davis's Birth of the Cool (1950). Despite Davis's top billing, this was in fact a collective project that drew together many players and arrangers/composers from the period: Miles Davis, Gil Evans, Gerry Mulligan, Lee Konitz, John Lewis, Gunther Schuller, and Johnny Carisi, although, according to Evans, Miles Davis was the musician who best represented this style of jazz. Issued only shortly after bebop had begun to establish itself, this recording offered an alternative aesthetic that was initially unpopular. It sold poorly and the band did not last long, but it eventually became a jazz classic.

Despite its beginning in New York, Cool Jazz later became identified strongly with West Coast jazz. Californian group The Dave Brubeck Quartet recorded the popular Cool Jazz album Time Out in 1959, which scored #2 on the Billboard "Pop Albums" chart. The Cool Jazz 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).

• List of cool jazz and West Coast jazz musicians

• Gioia, Ted, "Cool Jazz and West Coast Jazz", chapter in Kirchner, Bill, The Oxford Companion to Jazz, Oxford University Press US, 2005. ISBN 0-19-518359-2
• MacAdams, Lewis, Birth of the Cool: Beat, Bebop, and the American Avant-Garde, Simon and Schuster, 2001. ISBN 0-684-81354-8

• A History of Cool Jazz in 100 Tracks, edited by Ted Gioia (Jazz.com)
• allmusic Cool Jazz
• Forever Cool: Cool and West Coast Jazz on the Internet
• The Birth of the Cool 1927, by Len Weinstock.