Harmonic Major Applications

I just watched a Tom Quayle video on this topic that contains some good information but needed some translation before it made sense to me. I thought I'd provide the translation for anyone else who found it useful.

Here's the video:

Tom's fundamental point here is that in a given key there are three different diminished chords and each has a different function, and there's a Harmonic Major application for each one.

The first function he calls "tonic" diminished chords: in C that would be C dim, Eb dim, Gb dim and A dim. For this situation he recommends G Harmonic Major.

The second function he calls "flat nine" diminished chords: in C that would be Db dim, E dim, G dim and Bb dim. Here Tom wants us to play F Harmonic Major.

The third function he calls "nine" diminished chords: in C that would be D dim, F dim, G# dim and B dim. To most of us this looks like a tritone sub: D dim standing in for Db7, which is standing in for G7. The scale here is C Harmonic Major.

All this is tremendously confusing for those of us who like thinking in terms of scales rather than relative modes. So let's unpick it:

  • On "tonic" diminished chords, play Lydian Minor
  • On "flat nine" diminished chords, play Kosalam #5
  • On "nine" diminished chords, play Dorian b5

That may or may not seem simpler to you, but it seems simpler to me.

A question remains about whether this analysis makes any sense. The function of a diminished chord is usually defined by how it resolves, not by its relation to the key (which, if there's a diminished chord about, is probably about to change). The scales don't really match the diminished chord's tones very closely, so there's probably a simpler interpretation of what's going on and why these sounds "work". But that's for another day.