"East Coast jazz" during the 1950s.
The Lydian Chromatic Concept of Tonal Organization for
Improvisation. In the late 1950s, Bop was starting to run its course.
The chord changes were becoming more rapid and constrictive since
soloists had to play "follow the chords," and because after playing
their 32 measures the soloist had to play the same chord changes
again. Also, the style was about 15 years old, so the novelty and to
some extent, the creativity, was disappearing. Modal improvisation
used slower moving chord changes, and freed up the soloist to
improvise more based on scales, rather than the swiftly changing
chords of Bebop. The result was more melodic solos. The Miles
Davis group of the late 1950s popularized this music blurring the
genres of bebop & hard bop. John Coltrane spent the early 60s taking
this form to new heights.
Davis(1926-1991) series of singles recordings that later were released
on an album named The Birth of the Cool. The style uses the chord
changes, like bebop, but lacks the intensity or "emotional passion" of
hard bop. The music is more "laid back". The drummers in cool jazz
often use brushes, and hence, you do not get that driving rhythm from
the drums.