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In jazz, a constant structure is a chord progression consisting of three or more chords of the same type or quality. Popularized by pianists Bill Evans and Herbie Hancock, the combination of functional and non-functional chords provides cohesiveness while producing a free and shifting tonal center.
For example, the progression Fmaj7-A♭maj7-D♭maj7-G♭maj7-C13sus contains four major seventh chords (and one thirteenth chord), none of which are diatonic to the key of F major except the first.
In contrast, the vi-ii-V-I or circle progression from classical theory contains four chords of two or three different qualities: major, minor, and possibly a dominant seventh chord; all of which, however, are diatonic to the key. Thus diversity is achieved within a stable and fixed tonal center.