What separates cool jazz from other jazz styles and sub-genres? Which patterns are commonly found in it? Which are the key and defining characteristics of cool jazz? What makes a cool jazz song a cool jazz song?
Cool jazz is a style of jazz that arose as a sort of response to the growing popularity in bebop. Bebop featured fast-paced, syncopated, energetic music and frantic improvised solos. Cool jazz, in contrast, had a mellow and emotionally subtle "coolness" about it. There is still much melodic and harmonic complexity to it but you still won't see the same use of diminished and augmented chords that you see in bebop. Cool jazz often incorporates elements of classical music and is usually formally composed as opposed to being largely improvised. Another defining characteristic might be a musician's tendency to play behind the beat instead of driving it. Best examples: Dave Brubeck's Time Out, Bill Evans Trio's Explorations, and Miles Davis's Birth of the Cool.