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Diminished 7th chords in blues

Question by Erik Hlavaty
(Austin, TX)

Wondering if I have a tin ear:

In a simple blues progression in D (something like "Born Under A Bad Sign"), it sounds like I can replace both chords in the cadence this way -- to add some color: Adim7 for A7 (root note on 14th fret) and Gdim7 for G7 (root note on 12th fret).

The one theory book I have does not seem to cover this particular type of substitution, so I'm a little lost as to why it sounds like a valid substitution to me. Or, perhaps, it doesn't work and I need more ear training.

Thanks for your help.

Answer

Interesting question!

Traditionally in blues progressions (as you probably know), the diminished 7th functions as a "passing chord" between the IV and V chords.

E.g.

D7 / G7 / Abdim7 / D7 / A7 / D7

However, I've never come across actually substituting the IV and V chords with IVdim7 and Vdim7.

You mentioned using a dim7 rooted on the 14th fret (I assume you mean on the A string?). With a tonic of D major, that's actually the Abdim7 chord you're playing rooted a minor 3rd up from its natural root. Dim7 chords can move in minor 3rd intervals like this and provide the same effective voicing.

Now, applying this theory to the 12th fret dim7 you mentioned, the same chord can be positioned one half step up from the tonic D major, effectively giving us Ebdim7. I think this is actually what you're playing here, because this is another movement I've heard in blues, especially jazz blues.

e.g.

D7 / A7 / D7 / G7 / Abdim7 / D7 / Ebdim7 / A7 / D7

As we're flirting with jazz now, another typical addition would be the ii chord to give us a ii V I cadence, which flows naturally from that dim7 a semitone above the tonic...

D7 / Ebdim7 / Em7 / A7 / D7

So, in summary the following diminished positions are commonly used in blues progressions...

I - dim7 - ii (between the tonic and 2 chord)

IV - dim7 - V (between the 4 and 5 chords)

The test of whether you have a tin ear or not will come when you try to add some lead harmony over your progression. A flowing lead harmony can make even the most disjointed chord changes sound natural. If you can weave it through your diminished chords and connect them to the overall tonality of the I, IV and V chords, then there's really no such thing as right or wrong.

Any more questions just use the comments link below. Cheers!

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root note clarification
By: Erik Hlavaty

Thanks for the great post. Still trying to digest all that, but wanted to make one clarification.

The Adim7 and Gdim7 in this case have the root note on the G string -- 14th and 12th frets, respectively.

So, for the Adim7, the notes are A-C-D#-F# (I-3b-5b-bb7, I think), where the A is on the G string, 14th fret. Then, for Gdim7, the whole figure moves down a whole step to G-A#-C#-E.

Sorry I wasn't more clear on that.

Thanks, again.

Gotcha
By: Mike

Sorry I misunderstood. I can only say that I personally haven't heard the IV and V chords directly substituted with diminished 7ths before, but I'm obviously not hearing it in the way you are. That's the beauty of music, it's so personal.

The reason you won't see it covered in books is it's not standard practise. But again that doesn't mean it's wrong. The only real thing you need to avoid is harmony parts clashing (e.g. it is generally agreed that a major 7th and dominant 7th on the same root shouldn't be played together). As long as you reaffirm your diminished 7ths through your other harmony parts with purpose, then who knows, you might even be on to a kind of signature sound!

Keep experimenting with it.

Diminished 7th Chords in Blues
By: James Fortunato

root note clarification
Erik Hlavaty wrote:
By: Erik Hlavaty
Thanks for the great post. Still trying to digest all that, but wanted to make one clarification.
The Adim7 and Gdim7 in this case have the root note on the G string -- 14th and 12th frets, respectively.
So, for the Adim7, the notes are A-C-D#-F# (I-3b-5b-bb7, I think), where the A is on the G string, 14th fret. Then, for Gdim7, the whole figure moves down a whole step to G-A#-C#-E.
Sorry I wasn't more clear on that.
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What you're hearing Erik, is the V7(#9,#11,13) and the IV7(#9,#11,13) chords when you add those substitutions of Adim7 and Gdim7. You have the 2 roots of A & G and the C-D#-F# (from Adim7) and the A#-C#-E (from Gdim7) are the #9, #11, & 13 of both the A7(V7) and the G(IV7) respectively. They are upper structure tensions from those chords which are used all the time by Jazz artists. Your ear is doing just great I think. Try to figure out what chord scales you would use over those chords as an exercise to keep training your ear.
James Fortunato
Berklee Alumni 1975-79

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