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.
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.
See also 
- Rawlins, Robert (2005). Jazzology: The Encyclopedia of Jazz Theory for All Musicians, p.131. ISBN 0-634-08678-2.
- William G Andrews and Molly Sclater (2000). Materials of Western Music Part 1, p.227. ISBN 1-55122-034-2.