Birth of the Cool' became a collector's item, I think, out of a reaction to Bird and Dizzy's music. Bird and Diz played this hip, real fast thing, and if you weren't a fast listener, you couldn't catch the humor or the feeling in their music. Their musical sound wasn't sweet, and it didn't have harmonic lines that you could easily hum out on the street with your girlfriend trying to get over with a kiss.
Miles DavisThe music has gotten thick. Guys give me tunes and they're full of chords. I can't play them...I think a movement in jazz is beginning away from the conventional string of chords, and a return to emphasis on melodic rather than harmonic variation. There will be fewer chords but infinite possibilities as to what to do with them.
Miles Davis