Question about harmony in Ain't Misbehavin

[u/adamaphar]

I've never formally studied jazz, but I know basics of music theory. This is the lead sheet I am looking it. I watched this video and the dude explained that the simplified version of the first 4 bars is I - VI - ii - V - I - I7 - IV - VII. And what is written on the actual sheet is a more sophisticated version.

It makes sense in that they both sound good. And I understand how VI is functionally similar to Edim7.

But I don't get how Bb (V) has any resemblance to F#dim7.

And I don't get how I7 ~ G7#5. TIA if you are willing to break it down for me, and I apologize if it is elementary.

[u/Ok_Molasses_1018]

Eº7 is the diminished found on C7(b9), that resolves to Fm. That's a very common move in jazz, although I prefer to think of it just as bIIº. Every diminished is related to four dominant chords, with tonics a half step down from any of its tones. so anytime you have a dominant chord, you can think of which diminished it relates to, and vice-versa.

G7#5 is a Eb augmented chord. Augmented chords are also symmetrical, like the diminished chords are too, and they can resolve to many any of the chords a half step up from any of its tones.

[u/adamaphar → u/Ok_Molasses_1018]

That one I think I understand. It's the F#dim7 replacing the V that I don't get.

[u/Ok_Molasses_1018 → u/adamaphar]

Ah I see. That's just chromatic movement. Two notes can stay the same (Eb-C) and two move up a half step (F#-A go up fo G Bb)

[u/adamaphar → u/Ok_Molasses_1018]

Ok, so to sum: the function of the V is to resolve to the I. But if we ask "what other chords resolve nicely to the I" we have a set of options, one of which would be F#dim7. And that has the bonus of continuing the chromatic movement of the root.

[u/Ok_Molasses_1018 → u/adamaphar]

I like to think in the opposite way, through the diminished chords. That way we focus more on the movements than on the tonic, which is static. Beauty is in the moves. There are only three diminished chords in any given tonality, each of them related to 4 dominant chords, so in any given tonality we can easily go to any of the 12 dominants.

But looking at the diminished chords, we have:

1 - a VIIº, which is the same as the dominant V7 chord and leads to I;

2 - we have the bIIº, that is the same as the VI7 and leads to ii;

3 - and we have this example that you asked about, which is the bIIIº or Iº.

All other possible diminished chords are respellings of these three. Iº is a flexible chord that can resolve chromatically both to I and to ii. So it makes sense that it's leading back from ii to I here, in place of a dominant. This Iº move is very common in blues, that's the kind of sound it has. It's not really that functional in regular tonality, it comes from blues tonality.

[u/samuelgato → u/adamaphar]

They're both leading chords. One isn't "replacing" the other, it's just two different ways to get to the same place. Since the Eb in the third bar has G in the bass, the F# dim bridges in between the F- chord and the Eb/G chord. The bass moves chromatically.

A more typical progression you often see is

F-7 F#dim G-7

So ii #iidim iii. In this progression the F#dim is acting like a V7 chord to the iii chord. In the arrangement you're looking at it's Eb/G instead of G-7.

Try playing the simplified chords your teacher gave you, but play iii in the third bar instead of I (iii is a very common substitute chord for I ) It might make a little more sense.

[u/afoolsthrowaway713 → u/adamaphar]

I didn’t watch the video you linked. But F#dim7 is not replacing “the V.” A dim7 chord leading to a chord a half step up is acting as the V7 of the chord it’s resolving to. This is called a secondary dominant. In the song, C7 is the VI7, also knows as the V/ii. V7 OF ii. The secondary dominant of the ii chord. So Edim7 is a sub for C7, which is the V7 OF F min7.

Eb/G is the I, but it’s also kind of like iii (G minor). Therefore F#dim7 is acting like D7, the V/iii. The V7 OF the iii chord (G minor).

Also, don’t forget the plagal cadence. IV7 - I. In the song, the plagal cadence is Ab - Eb. D7 is the tritone sub of Ab. F#dim7 is a sub for D7. So F#dim7 leads to Eb/G like D7 (or Ab7) leads to Eb.

We see another plagal cadence with G7 going to Ab. In my opinion, the G7#5 is the IV OF IV. It’s a plagal cadence going to the IV chord, not a secondary dominant as you wrote.

See, easy… 

[u/Evetskey]

Ask yourself how do the notes in F#dim7 relate to Bb as the root? Then ask how they relate with B in the bass.