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The other day I was playing guitar and messing around with adding various embellishments to a blues I-IV-V progression in the key of A. One I particularly enjoyed was substituting the D (IV) chord for a G7. I'd like to understand the theory behind this. Thanks!

Here is a recording:

https://soundcloud.com/seymour-frishberg/g7sub/s-nLxx8

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  • Hm, the SoundCloud integration can't seem to work. – leftaroundabout Nov 15 '19 at 23:30
  • For the question: Ⅰ - Ⅳ - Ⅴ is kind of unidiomatic for blues in the first place. – leftaroundabout Nov 15 '19 at 23:33
  • "substituting the IV chord ... for a G7 instead of D". Do you mean "substituting G7 for the IV chord D"? or "replacing the IV chord ... by a G7 instead of D"? Laurence Payne seems to have answered on that basis but I think it'd be better if the question title were clearer. – Rosie F Nov 16 '19 at 8:15
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If we're trying to fit everything into a functional 'cycle of 5ths' system, IV is actually a substitute for ii, V of V. ♭VII 'works' because it has two notes in common with ii.

♭VII7? Well, in a jazz/blues environment, you can add a 7th to just about any chord and it won't sound bad!

There are similarities with the 'backdoor progression' where ♭VII is used as a substitute for V7. Again, it works because ♭VII has two notes in common with V7.

But beware of trying to hook EVERY progression onto an underlying functional ii, V, I. 'Giant Steps' took that sort of thing about as far as it could go.

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  • Ah, so basically the G7 works because it shares notes with Bm and it's okay to make it a 7 because ＪＡＺＺ. Are there any other examples of using the ♭VII in this way or similarly? Thanks for your answer! – Seymour Frishberg Nov 16 '19 at 0:14
  • Oh! Or could I also think of that chord as being from a particular mode? – Seymour Frishberg Nov 16 '19 at 0:16
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    You can label most things as being from a mode. Not sure it proves anything much. It's not in one scale, but it's in another scale. So? – Laurence Payne Nov 16 '19 at 2:16

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