B♭ Phrygian — 7th chords
Four-note stacks of thirds on each degree — root, 3rd, 5th, 7th. The added 7th gives jazz, gospel, and modern classical voicings their distinctive colour.
All 7 seventh chords in sequence
Degree 1 · i7Bbm7
Degree 2 · IImaj7Cbmaj7
Degree 3 · III7Db7
Degree 4 · iv7Ebm7
Degree 5 · v°7Fm7b5
Degree 6 · VImaj7Gbmaj7
Degree 7 · vii7Abm7