Modal Music is not so much a style of jazz as it is a structure. Before the advent of modal music in the '50s, solo improvisations were based around the specific key of a piece — that is, its tonal center, the starting point to which its melodies and chord progressions would return for a feeling of resolution or completeness. Modal improvisations, on the other hand, were based on modes, or scales — but not just the typical major and minor scales familiar to nearly all musicians. The most commonly used modes did relate to major scales, though; each note in the scale was also the first note of a new mode, which would incorporate all of the notes in the original major scale, but sounded different because the new starting point rearranged the order of distances between notes. Thus, if a musician were improvising over, say, a D chord, he could essentially choose any key center whose corresponding scale included the note D (unless the composer dictated specific modes to be used in the solos). The new order of distances between notes could produce very different moods — for example, even though the C major and A minor scales use exactly the same notes, the former sounds bright and happy, whereas the latter is more melancholy. Because the feeling of the music came from those note choices, chord progressions in modal music were usually kept very simple, since too much motion would have allowed little time to fully explore the mode(s) that had been selected. The results often had a meditative, cerebral feel, but weren't quite as mellow as, for example, West Coast-style cool jazz. Modal music had a subtle tension produced by the fact that the solo lines, while melodic, didn't always progress or resolve exactly as the listener was accustomed to hearing; plus, every time a new mode was introduced, the tonal center shifted, keeping the listener just off balance with a subtle unpredictability. Miles Davis was the first jazz musician to improvise and compose according to modal structure; his Kind of Blue is the definitive modal jazz album, and two of his sidemen on the record — John Coltrane and Bill Evans — later went on to become modal innovators in their own right. The freedoms of modal music helped pave the way for the rabid structural experimentation of avant-garde jazz, which would begin to take shape toward the end of the '50s.