Determing Scale/Mode Discussion

I posted this in response to a fellow student's question at Berklee, which was, more or less, "I know that using the V7 chord to determine scale works for major modes. How does it apply to other modes/scales?"

My reply, which was evidently well received:

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I'll take a stab at this, as I've toyed around with it a bit; the same logic you mention seems to apply. Look for the structures that are characteristic of the scale, particularly as they relate to the dominant and tonic chord within it (if they are present...they aren't always).

Major:
I ii iii IV V7 vi vii(b5)

Minor:
i ii(b5) III iv v VI V7

Melodic minor:
i(maj7), ii, III(#5), IV7, V7, vi(b5), vii(b5)

Harmonic minor:
i(maj7), ii(b5), III(#5), iv, V7, VI, vii(b5)

So, you look at the chord structures, then determine the possibilities. For instance, say you got into a song that vamps on an E7 for the lead for 16 bars. You might say, "that's a V chord of A major. So my improv is A major" (modally, E mixo, 5th mode of A major). That's correct, albeit predictable. Nothing wrong with that though, a good mixo solo is a great thing.

BUT...E7 is also the V chord of A harmonic minor. So, to spice it up, I'll start in E mixo, and hit a tone common to A harmonic minor (G# is a good one, it's the third of E7, and the seventh of A harmonic minor), then play in E phrygian dominant (the 5th mode of A harmonic minor), then before the end of the lead, pivot off the G# again and come out in E mixo. Very dramatic and sets you apart from the penta minor crowd.

Single chord vamps have a lot of possibilities though. Here's something more structured:

i, VI, V7, i.

It's got a minor chord as the resolution; lot of possibilities there. But, I see it has a major chord on the VI, and a dominant chord on the V7. Look at the diatonic structures we know; that's harmonic minor (play that progression; you'll hear it loud and clear).

Alter it to this:

i, vi(b5), V7, i

And it's not harmonic minor anymore, because harmonic minor doesn't have a min7b5 chord on the vi...melodic minor does though. If you play this one, again, you'll hear it loud and clear (and might recognize it as a VERY common jazz progression...go 4 beats on the i, and 2 beats on the vi(b5) and V7).

So the original thinking is valid, you just keep applying it.

But what if no tonic, and/or dominant chord, is visible in the arrangement? For instance, one I frequently see the hippies get wrong:

Amaj, Bmaj (Fire on the Mountain).

I see this played in either A or B Ionian all the time. Both are basically wrong, because neither chord is the tonic. Two major chords in a row...that could be a few things, but straightforwardly, that's a IV-V progression. So, E f g A B...E major. To use the roots of the chord, use either the fourth or fifth mode; A Lydian or B Mixolydian. The hippy-in-the-know knows that Lydian was Jerry's favorite mode (the #4 is a huge part of Jerry's spacey lead style), so, you go for A Lydian. It sounds great. Don't try to explain it at the jam though.

Another example...two Dom7 chords in a row.

A7, B7.

What scale, diatonically, has two dom7 chords consecutively?

Melodic minor (on the IV7 and V7). So, if you hit a song that is just two dom7 chords like this in a row (or is mostly those two chords), you know that you E melodic minor is a possibility, even though no tonic chord is visible.

Everything is subject to interpretation, and the ear rules all, but as far as I understand it, this is the basic mechanics of making these kinds of determinations. Powerful stuff for the improviser.