The Diminished Hypermode and "Expanded Harmonic Minor"

We can think of the Harmonic Minor scale as a minor triad with a diminished seventh chord a semitone below it. In fact, I'd guess this is how the scale originally came about, and you can hear this relationship frequently in the music of Bach's time. This post is about an expanded version of that idea.

It will turn out to be helpful to think of the minor chord as a minor-major seventh (i.e. a minor triad with a major seventh on top). Here's CmM7 (C-Eb-G-B) plus B° (B-D-F-Ab), which together cover C Harmonic Minor (C-D-Eb-F-G-Ab-B):

This is already a useful way to do things. The CmM7 is more of a twentieth century harmony that leads my ear, at least, into some different territory from those linear Harmonic Minor lines that can be a bit baroque-and-roll.

But if you can approach CmM7 from below, why not approach it from above instead? That would mean combining CmM7 with Db° (Db-E-G-Bb, excusing my enharmonic spellings which will be useful in a moment). The result is very exotic:

The E (really Fb) in Db° is the major third of CmM7, which is a strong "avoid note", which is music school language for "a note you should play because it's spicy and interesting". What's more, the Bb (really Abb) in Db° kind of spoils the "leading note" effect of the B, making it a bit ambiguous whether you're trying to evoke a clear Harmonic Minor kind of cadence or doing something more modal. These features make this a deliciously strange combination of notes. When forming melodies with them, though, you can always think "diminished chord resolving down a semitone" and it will lead you to quite Harmonic-Minor-flavoured results.

So if both of these are interesting, why not play both? Combining the notes of B° and Db° produces B-Db-D-E-F-G-Ab-Bb, which is B Whole-Half Diminished or Db Half-Whole. If we think of this as notionally still "in C", we're looking at the unique hypermode of Half-Whole:

This is what I think of as "expanded Harmonic Minor". You can play Harmonic Minor lines with it, or you can twist it using the Cm-plus-Db° idea, or you can drop the C entirely and drift over the top of it with purely diminished language. And, of course, you can move freely between all these things, which is really the point.