Chords in Bb Dorian

Bb Dorian is built from the notes Bb, C, Db, Eb, F, G and Ab. Its scale carries 4 flats. Harmonising that scale in thirds gives seven chords — 3 majors, 3 minors and 1 diminished — starting on Bbm as the i. The chord on the fifth degree is Fm rather than a major V, so Bb Dorian has no leading tone and resolves by step and colour instead of by dominant pull — the sound that separates it from a plain Bb major.

Beyond those seven, Bb Dorian commonly borrows Bb (I), Dm (iii), Eb (IV), F (V) and Gm (vi) from the parallel major — the mixture chords that give a progression somewhere unexpected to go.