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