IV major to IV minor

[frdagaa]

In some blues forms (I think of them as gospel forms, though that may be incorrect), the IV chord changes from major to minor. In other forms the IV leads to a #IVdim7 at the same place. Both are usually followed by the I. So is there some functional equivalence between a IV minor and a #IVdim on theoretical grounds?

eg:
I7 III7 IV ivm I VI II V etc
I7 III7 IV #IVdim


E7 G#7 A Am E
E7 G#7 A Bbdim E

[JonR]

Originally Posted by frdagaa

In some blues forms (I think of them as gospel forms, though that may be incorrect), the IV chord changes from major to minor. In other forms the IV leads to a #IVdim7 at the same place. Both are usually followed by the I. So is there some functional equivalence between a IV minor and a #IVdim on theoretical grounds?

eg:
I7 III7 IV ivm I VI II V etc
I7 III7 IV #IVdim


E7 G#7 A Am E
E7 G#7 A Bbdim E

Your choice of III7 is unusual. Much more common to see I7 in that situation:
E E7 A Am, or E E7 A A#dim7
III7 would be a secondary dominant (V/vi) and the following IV is a deceptive cadence.

Anyway...
IV minor and #IVdim are quite different.
Essentially they each provide a chromatic passing note.
In key of E, the line on A-Am-E runs C#-C-B; while on A-A#dim7-E it runs A-A#-B.
And of course, the lines in each case extend back to the E chord:
E-D-C#-C-B, or
E-G#-A-A#-B.

In fact, it's common for a #IVdim7 to be followed by a 2nd inversion tonic (A#dim7 - E/B), prior to a V chord and a return to root position tonic. In a sense, you could interpret A#dim7 as a sub for a secondary dominant, F#7 (V/V), with the following E/B being a kind of suspension on the B bass - hence the likely B-E chords to follow.
You might (in jazz) also get a cycle of 5ths, with an extended rising line:
|E - - -|E7/G#- - -|A - - -|A#dim7 - - - |E/B - G#7/B# -|C#7 - - -|F#7- B7 - |E.

A jazz variant on the the minor iv chord is to transform it into a bVII7, leading to the so-called "backdoor" cadence (bVII-I).
In key of E:
E-E7-A-D9-E(maj7)

In rock, OTOH, the minor iv may become a major bVI:
E-E7-A-C.

[JohnM]

Originally Posted by JonR

Your choice of III7 is unusual. Much more common to see I7 in that situation:

Some people mistakenly hear the bass note in an inverted I7 (with the 3rd in the bass) as a III7. (fwiw...may not be the case for the OP.... I dunno)

[frdagaa]

The III7 after the I may not be standard blues, but does happen and is fairly typical of a particular genre. Play

E////G#7 //// A //// Am //// E //C#7 // F#7 // B7// E//// B7////

and it should sound quite familiar.

I agree with the voice-leading analysis. I guess that's mainly it. But it just strikes me that I might be missing some other theoretical equivalence of a iv minor and a #iv dim.

Interestingly, the various forms Rhythm Changes takes also points to some equivalence in bars 5-6 (/ Fm Bb / Eb Edim/ vs. (/ Fm Bb / Eb Ebm/)