Please re-read Diminished perspectives .... chords that is for updated edits which I hope will add some sanity to my disjointed take on the dim 7 chords from last time.
From Robert O.G. who (paraphrased) thinks of the diminished passing chord (iiimin7-biiidim7-iimin7) as an auxiliary diminished. More like (Imaj7/III-idim7/biii- iimin7) makes more sense. I think that has utility and is good to keep in mind when approaching improv or arranging. In fact the diminished chord comes after the iiimi7 and is leading to the ii mi7 and by virtue of that can be classified as a passing dim.
This thought was taken up by Marcus W. (many apologies) who confirms: Re: iiimin7 equalling Imaj7- This makes sense in both realms. Classical tonality says that the iii min is too weak and therefore a manifestation/
prolongation of 1maj, so the dim. chord functions as passing.
My response was to affirm the idea of direction of the progression. Where is it going... in this case from iiimi7—iimi7 with the biii dim7 between i.e. iiim7—biiidim7 –iimi7...
Ima/iii—biii dim passes to iimi7 and the relation that biii dim has to iiimi7 is not considered auxiliary because it no longer relates to iiimi7 but relates and travels to ii ...I think that principle is valid. The auxiliary idea of biii is a moot point. But, the auxiliary dim 7 idea applied to the iiimi7 (or Ima7/iii) is also good to keep in mind I think.
I'd like to repeat the idea of Auxiliary Dominants. I don't hear people in the know defining these chords that way but it appears to have some wheels... (as they say) at least from the perspective I'm developing.
OK: C6—B7—C6 is a classic maneuver and is derived from C6—Cdim7—C6. Cdim being the derivation of B7(b9) etc. This has "release" C6 (tonic) i.e. at rest !! traveling to B7, a chord out of the tonality of C major— .... and then resolving that "out of key sense" back to C6...
This is how all 4 of the Auxiliary Dominants work.... they are derived (per say) from the root diminished chord (Cdim 7 in this case)..
in C or C6:
C6—B7—C6 acts like a lower neighbor chord
C6—F7—C6.... use in blues and boogie etc.... with utility in jazz rock too..
C6—Ab7—C6 ...(as in Bye Bye Blues)...forms a basis for use in so-called augmented 6th chords ..German 6h etc.
C6—D7—C6.... offers a milder form of tension and release but as D7(b9) this chord is derived essentially from C dim7..
More on this later...