ok.. I don't get it. I read about it and I can figure it out (mostly) but I don't hear it. How do I use this chord substitution? If I understand what I have read if I am playing a C7 chord I can sub in a G-flat/F-sharp 7 in its place?
If anyone has the wherewithal to explain.. or better yet, record an example of this I would be really appreciative. It sounds (no pun intended) useful, but I can't seem to get my head around it.
Yes and no... which is actually the proper answer to almost every chord substitution question :)
For a dominant chord, yes you can substitute a dominant chord that's a b5 higher. So, for C7, you can use Gb7. Chord tones C-E-G-Bb become Gb-Bb-Db-Fb/E, so you retain the critical tones of the third (E/Fb) and seventh (Bb). The problem comes when that C7 resolves to something else...
The usual resolution of a dominant chord is to a root a fifth lower, so that C7 generally wants to go to F something, and the Gb7 will work because you're moving the whole thing down chromatically - Gb7 goes to F, down a half step.
If the original chord, C7, should happen to move down a half step to a B something, the b5 substitution still works - now your Gb7 resolves nicely to B in a V-I cadence (the Gb7 functions as the enharmonic F#7).
In a few tunes, the C7 moves to a Gbm chord. The b7 substitution for C7 works in those situations too... the root and fifth stay the same, and the 3rd moves down a half step. Thinking in four-part harmony, the b7 moves up a whole step to the new root. This progression is pretty rare, though.
If your original C7 moves to a root other than F, B, or Gb, scrap the b5 substitution for something else. You'll have a devil of a time making it resolve so it sounds like you intended the change, and there are plenty of other substitutions available.
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