Key Center and Roman Numerals

You've got a couple of tricky things going on.

First, minor keys have a LOT of latitude. In a major key, the ii is a minor (or m7), because the notes of the major scale are always the same. But in a minor key, there are LOTS of different scales we can use, and each scale will harmonize to create different chords. So when you look at Am7 (A-C-E-G), you're assuming it will be m7b5 to fit the Eb in the Gm scale.

Look at the G natural minor scale (G-A-Bb-C-D-Eb-F-G) and you're right... it should be Eb. But what if we used the G ascending melodic minor (G-A-Bb-C-D-E-F#-G), or the G Dorian (G-A-Bb-C-D-E-F-G)? We'd end up harmonizing the ii to be a m7 in both cases!

Scales can be freely mixed. In fact, that's what the G melodic minor does - it's one scale going up (G-A-Bb-C-D-E-F#-G) and a different one going down (G-F-Eb-D-C-Bb-A-G). So we can mix and match the chords to suit what the melody is doing at the moment.

The second tricky thing is a substitution. We expect a V-i cadence, which would be D7-Gm if you were doing something simple. The tension in D7 comes from the tritone between F# and C. Instead we see Ab7, which has a tritone between C and Gb... notice that Gb is enharmonic to F# - the Ab7 chord is actually a tritone substitution for a V7.

So how you apply Roman numerals will be tricky. Harmonically, that's a V7-i. If you're just trying to see the overall shape of the progression, that's how I'd label it. But if you're trying to see the actual chords used so you can use the same sequence someplace else, I'd label it bII7.

Does that help?

Oh! I just thought of natural minor scale only, not melodic or dorian scale etc. Now I fully understand with your very kind and professional explanation.

Regarding dominant 7th chord substitution, yes! D7 and Ab7 share the same b7th and 3rd tone. Now I remember this is "tritone substitution"!

Thank you so much, NoteBoat.

Taesoo