Chords in Bb Mixolydian

Bb Mixolydian is built from the notes Bb, C, D, Eb, F, G and Ab. Its scale carries 3 flats. Harmonising that scale in thirds gives seven chords — 3 majors, 3 minors and 1 diminished — starting on Bb as the I. The chord on the fifth degree is Fm rather than a major V, so Bb Mixolydian has no leading tone and resolves by step and colour instead of by dominant pull — the sound that separates it from a plain Bb minor.

Beyond those seven, Bb Mixolydian commonly borrows Bbm (i), Cdim (iiø), Db (bIII), Ebm (iv) and Gb (bVI) from the parallel minor — the mixture chords that give a progression somewhere unexpected to go.