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I've been playing with diminished chords as 'V' in ii-V-1 progressions. I often use one of the three diminished chords in a ii-V-1 progressions - I sharp the 5th note of the scale and hang a diminished over it. So in C major I would use G# diminished for V which is really a rootless G9 so I can understand its dominant function.

But I've just been trying Eb diminished and it sounds dominant too but I don't understand why. (I've been playing 'Almost like being in love', and Eb diminished sounds great over 'being'). I then tried the only remaining diminished chord and it too seems to provide a adequate 'V' function, although it's not as nice as the other two in this context.

So, I'm wondering. Can you use any diminished as 'V' anywhere and get away with it, and is my use of Eb diminished over 'being' (i.e. V) a standard substitution? And how does it work?

Cheers, David

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@AmericanLuke The fifth scale degree is diminished in Locrian mode; the V triad is diminished in Phrygian mode. –  Bradd Szonye May 28 at 23:22

2 Answers 2

Since you're referring to there being only three diminished chords, I'm going to assume you mean diminished seventh chords. In that case, what you're calling a G#dim, I would actually call a Bdim7, and spell (B D F Ab). This has the benefit of making the chord root be a scale degree (the seventh scale degree of the major scale supports a diminished chord). The Ab also makes it a bit clearer that it functions as a rootless G7(b9) chord, as you point out. If you were to spell it as (G# B D F), I'd expect a resolution to some form of A chord.

In general, the root of a dim7 chord can be treated as the major third of a rootless dominant 9th. That makes it the leading tone of some scale, and it wants to resolve a half-step up. From your question, I think you understand this point already. However, because the chord is symmetrical, any of the four tones could be viewed as its root, which gives four possible resolutions. I like to think of it like a "revolving door" of tonality.

But your real question is about what you're calling Eb dim (which would be spelled Eb, Gb, Bbb, Dbb). I'd probably consider this either an F#dim7 (F#, A, C, Eb) or an Adim7 (A, C, Eb, Gb) depending on context. I'm not familiar with the song you mentioned, and I'm not at a keyboard at the moment, so I'm having a hard time seeing how this could substitute for a dominant. However, it looks like it would make a great predominant substitution. I'll edit this answer when I get a chance to try it. So I guess I haven't answered your question yet.

I'm not sure about the third dim7 chord either, but since it includes a bII scale degree, maybe it acts like a modified tritone substitution? I'll have to check that out later as well.

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I'm not sure whether you're describing full diminished seventh chords (eg. A C Eb Gb) or diminished triads (eg. A C Eb), but both are very closely related to dominant seventh (V7) chords (eg. F A C Eb, dominant seventh in Bb Major).

A diminished triad is the same as a dominant seventh chord without its root. As a diminished seventh chord is symmetrical, consisting of notes that are all three semitones apart, it can be converted to four different dominant seventh chords by dropping any of its notes by a semitone. For instance, the diminished seventh A C Eb Gb can become Ab7 by dropping the A a semitone, B7 by dropping the C, D7 by dropping the Eb and F7 by dropping the Gb. Also, a full diminished seventh chord is the same as any of four 7b9 chords without a root.

It should be pointed out that changes of enharmonic spelling are often necessary to create the correct theoretical type of V7 chord from diminished seventh chords.

An Eb diminished seventh chord would seem to function more like a secondary dominant in C Major (like a D7b9). This should feel like it wants to pull towards the dominant, G, but this "double" pull, towards the tonic from the dominant of the dominant may still make it work okay as a dominant substitution, simply because your ear feels the true dominant and then tonic that you "want" to go to. I bet it would sound even better with a really cheeky little G7 chord after it though...!

Finally, another reason this Eb diminished chord may work well as a strong approach to the tonic triad, is because the Eb (or D#) and the Gb (or F#) want to move upwards by semitone, almost like leading notes, to the third and fifth of the tonic triad. And, the other diminished seventh triad (that isn't on Eb, or on D, the equivalent of the dominant seventh) is on E. This has Db which will "pull" downwards towards the tonic, C.

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