Bob,

I think the solution is, as a number of people have already mentioned, that the diminished chords are really a variation of chord vi (Am). It comes about because Adim7, Cdim7 and Ebdim7 are all the same chord. (This is what I tried to explain in my first post in this thread. Then I got overly self-indulgent in theory and probably scared people off!)

My take on it is that the REAL progressions are ...

C | Adim7/C | Dm7 | G7

C | Adim7/Eb | Dm7 | G7

They make sense as simply different inversions of the Adim7 chord. From a music reading point of view, though, it's much easier to read Cdim7 or Ebdim7 rather than Adim7/C and Adim7/Eb, respectively.

Near the end of my earlier post, I put a link to an audio file where different versions of chord vi are played (including the above diminished chords). To my ear, all the chords sound like they fulfill the role of a version of chord vi. It would be interesting if they sound like that to you too.

With Peter's last chord progression,

F | F#dim7 | C/G | A7

My thoughts are as follows ...

C/G is often thought of (classically, at least) as a G chord with a suspended 4th and 6th rather than a C chord.

From this perspective, F#dim7 is a substitute for D7 in the same way that Bdim7 is a substitute for G7. In this regard, Fdim7 can be thought of D7b9 without the bass. For this reason, I see the progression as a variation of D7 to G.

All the best,
Noel

EDIT: Corrected wrongly named chord ii to chord vi

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