Chords in Bb Locrian

Bb Locrian is built from the notes Bb, Cb, Db, Eb, Fb, Gb and Ab. Its scale carries 7 flats. Harmonising that scale in thirds gives seven chords — 3 majors, 3 minors and 1 diminished — starting on Bbdim as the iø. The chord on the fifth degree is Fb, a major bV, so Bb Locrian has the leading tone that makes a Fb-to-Bbdim cadence sound final.

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