Why the best-selling jazz album of all time is so great.

The clearest example of its novelty is a piece, composed (without credit) by Evans, called "Flamenco Sketches." At most jazz sessions, the sheet music that the leader passes around to the band consists of "heads"—the first 12 or so bars of a tune, with the chords notated above. The band plays the head, then each player improvises on the chords. But for "Flamenco Sketches," Evans had jotted down the notes of five scales, each of which expressed a slightly different mood. At the top of the sheet, he wrote, "Play in the sound of these scales."

For the band's two saxophone players, John Coltrane on tenor and Julian "Cannonball" Adderley on alto, it was a particularly bizarre instruction. Both were astonishingly adept improvisers, but they built their creations strictly on chords, Adderley as an acolyte of Charlie Parker (with a gospel-infused tone), Coltrane as an almost spiritual explorer, searching for the right sound, the right note, mapping out his voyage on charts of chords, piling and inverting chords on top of chords, expanding each note of a chord to a new chord, not knowing which combinations might work and therefore trying them all.

A few months after the Kind of Blue sessions, Coltrane led his own band on an album called Giant Steps, which pressed this quest to its ultimate degree—literally:

Giant Steps marked the end of the bebop frontier; Coltrane knew this, and, afterward, would go in a whole new direction, less tethered to structure, more "free," than even Russell's concept envisioned. But on Kind of Blue, especially "Flamenco Sketches," he took his first—and most lyrical—step out on that brink:

The departure from bebop is clear from the album's opening tune, "So What," which would emerge as this new sound's anthem. Evans describes it on the album's liner notes as "a simple figure based on 16 measures of one scale, 8 of another and 8 more of the first … in free rhythmic style." (Loose as it is, this was more structured than some of the pieces. Evans writes that, for "Flamenco Sketches," the improvisations on each scale can last "as long as the soloist wishes.") In this 30-second clip from "So What," Davis improvises on a single scale for all but the last few seconds, when Evans signals a shift to a different scale:

Compare this with "Freddie Freeloader," the album's only conventional blues. (For this track alone, Miles let his usual pianist, Wynton Kelly, a straight blues-and-bebop keyboardist, sit in for Evans):