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Structurally, it's similar to the early bebop tunes that Davis played with Parker in the mid-1940s, the melody latched to the pianist's chord changes, which occur nearly every bar, as in this 1946 Parker recording of "Ornithology" with Davis as sideman:
Now contrast these conventional bop pieces with the most fully developed piece of "modal" jazz" on Kind of Blue, called "All Blues":
It has the same feel as the other blues tunes, but listen closely: The horns, blowing harmony in the background, are playing the same notes in each bar; they're not shifting them to follow the chord changes; there are no chord changes. It sounds (hence the album's title) kind of blue.
So Kind of Blue sounded different from the jazz that came before it. But what made it so great? The answer here is simple: the musicians. Throughout his career, certainly through the 1950s and '60s, Miles Davis was an instinctively brilliant recruiter; a large percentage of his sidemen went on to be great leaders, and these sidemen—especially Evans, Coltrane, and Adderley—were among his greatest. They came to the date, were handed music that allowed them unprecedented freedom (to sing their "own song," as Russell put it), and they lived up to the challenge, usually on the first take; they had a lot of their own song to sing.
The album's legacy is mixed, precisely for this reason. It opened up a whole new path of freedom to jazz musicians: Those who had something to say thrived; those who didn't, noodled. That's the dark side of what Miles Davis and George Russell (and, a few months later, Ornette Coleman, in his own even-freer style of jazz) wrought: a lot of noodling—New Age noodling, jazz-rock-fusion noodling, blaring-and-squealing noodling—all of it baleful, boring, and deadly (literally deadly, given the rise of tight and riveting rock 'n' roll). Some of their successors confused freedom with just blowing whatever came into their heads, and it turned out there wasn't much there.
Another appealing thing about Kind of Blue, though it's also a heartbreaking thing: There was no sequel. Soon after the recording date, the band broke up. Evans formed his own piano trio; Adderley went back to playing gospel-tinged bop; Coltrane (after making Giant Steps) took his own road to freedom; Davis, too, retreated to earlier forms for the next few years, until he formed his next great band, in the mid-'60s, with younger musicians who pushed him on to more adventurous experiments.
Kind of Blue is a one-shot deal, so dreamily perfect you can hardly believe someone created it. Which is why it remains so deeply satisfying, on whatever level you experience it, as moody background music or as the center of your existence. Listen to it 100 times or so, and you still marvel at its spontaneous inventions; now and then, you'll even hear something new.
Correction, Aug. 19, 2009: The article originally stated that a scale consists of 12 notes, which is true for chromatic scales (scales with all the notes—natural, flat, and sharp), but since Russell was talking about scales or "modes" that sound different from one another (meaning they include at least some different notes), this can be true only of scales with eight tones or eight notes. ( Return to the corrected sentence.)