A♭ Dorian — 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 · i7Abm7
Degree 2 · ii7Bbm7
Degree 3 · IIImaj7Cbmaj7
Degree 4 · IV7Db7
Degree 5 · v7Ebm7
Degree 6 · vi°7Fm7b5
Degree 7 · VIImaj7Gbmaj7