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Question by Nick Champlin
(Alamogordo, NM, US)
Question: Hello my question is about the use of the Dim7 (diminished 7th) chord/arpeggio. I understand that, in the terms of the major scale (in this case G major), half diminished or m7b5 chords can be played over/with the "vii" or F# chord or along with the F# locrian mode because of its interval relation to this mode. Since all the notes in the F#m7b5 (F#-A-C-E) chord are within the G major scale it goes very well together.
I have gone through countless scales and modes trying to work this Dim7 in. So here is my question how or where could I use the Dim7 with out sounding too "outside" the normal Major/Minor keys?
Also what would be the most ideal chord to use this over (e.g. I-ii-iii-IV-V-vi-vii)?
Firstly, for anyone reading who has not yet learned the distinction between half diminished and diminished 7th chords, visit the diminished guitar chords lesson.
Nick, it sounds like you've grasped the intrinsic relationship between half diminished chords, the vii (7th) scale degree and the Locrian mode so nice work!
Diminished 7th in harmonic minor
If we were to apply diminished 7th chords to a scale in a similar way we do half diminished chords to the major scale, we would use the harmonic minor scale. This is the scale in which the diminished 7th "naturally" occurs (where it can be considered an "inside" chord within harmonic minor based progressions).
With harmonic minor as our parent scale, diminished 7th chords naturally occur on the 2nd, 4th, 6th and 7th degrees. That is the ii, iv, vi and vii chords of harmonic minor.
The reason it appears on more than one position is due to the interval relationship of the dim7 chord. It can be broken down into minor 3rds (one and a half steps between each interval).
Therefore, we can take the dim7 in it's ii position and move up a minor 3rd to the iv position of harmonic minor. Another minor 3rd lands us on the vi degree, and so forth.
E.g. Bdim7 / Ddim7 / Fdim7 / Abdim7 - all use the same notes believe it or not!
Even when not used in the context of harmonic minor, this minor 3rd interval movement still applies.
Using diminished 7th in chord progressions
Dim7 is actually a very versatile chord. It can simply replace a standard m7b5 (half) diminished chord or it can be used between whole step chord intervals...
Take this common jazz progression
Cmaj7 / Ebdim7 / Dm7 / G7
Cmaj7 is obviously the I (tonic) chord. Ebdim7 slots into the whole step gap between where the ii and iii chords would naturally be.
C / F / G / Abdim7 / Am / G / F
So there the dim7 chord bridges the V and vi chords, since there's a whole step interval between them.
Now, while this is fine for chord progressions, the modes you'd normally use in the context of this diatonic chord scale (I ii iii IV V vi vii) won't be compatible with the dim7 used in these "outside" positions. This is where you need to apply a separate scale to some chords.
The scale most commonly played over dim7 chords is the... diminished scale, which is different from many scales you'll learn because it contains 8 notes rather than 7.
Hope this helps. Please ask any further questions using the comments link below.