Western harmony traditionally relies upon a tonal key centre with related chords and cadences.

Modal harmony, however, takes a chord and corresponding scale (or mode), where it may remain for some time or move to another, possibly unrelated mode.

Jazz musicians began to experiment with composing in this way in the late 1950s, inspired by theoretical work of George Russell. Miles Davis’s ‘Milestones’ (from the album of the same name) and ‘So What’ (from Kind of Blue) are key early examaples, where players improvise within a mode for extended periods; John Coltrane would intensify this approach with his classic 1960s quartet.

During the mid-1960s composers like Wayne Shorter, Herbie Hancock and Joe Henderson further utilised ‘non-functional harmony’, writing music that moved more quickly between chords and corresponding scales in sometimes surprising ways.