I have to give one of my former instructors credit for this one. Dr. Peter Gries, who until his recent retirement was the music department chairman at my old alma mater, spent quite a bit of time on this in advanced theory. He was also the first proponent of Shenkerian Analysis I had ever met – and had this incredibly radical idea that music analysis should be based on what one hears rather than what one sees on the page. The latter he referred to as Augenmusik (“eye-music”).
Anyway…most jazz players and arrangers think of a diminished seventh chord, or a stack of minor thirds, as a color chord at worst and at best as a substitute for the dominant 7th, or V7 chord. It is true that the seventh chord is useful as a functional chord leading to something else. And, when one places a bass note under a diminished seventh corresponding to the note a whole step up from the root of the chord, it changes its personality altogether. That bass note turns the diminished seventh into a dominant seventh with a flatted ninth, based on that same bass note.
So, a C diminished seventh chord with a D in the bass becomes a D7(b9), a Db diminished seventh with an Eb in the bass becomes an Eb7(b9), a D diminished seventh with an E in the bass becomes an E7(b9) and so forth.
Listen to it here.
Note: while the real theory jocks who are into “Augenmusik” would take issue, try to ignore the way the chords are spelled in the examples for this post and focus on how they sound.
By the way, did you know that there are really only three diminished seventh chords? They may have different names – and according to the music theory jocks, different functions – but a C diminished seventh and the Eb, Gb/F# and A diminished seventh chords all have the exact same pitches. The same goes for the diminished seventh chords built on Db/C# (Db/C#-E-G-Bb) and D (D-F-Ab-B).
Don’t believe it? Hear it for yourself.
Incidentally, the same is true of augmented chords, or stacks of major thirds (which I’ll deal with in a future post).
Getting back to diminished seventh chords: the most amazing thing about them (and this is what Dr. Gries explained to us) is that by simply dropping any pitch one-half step, it can be turned into the V7 of any key to which you would care to modulate.
Allow me to demonstrate:
In short, the diminished seventh chord is the Universal pivot chord. The technical name for this is “common tone diminished chord modulation.” Can you see why?
Bottom line: if you are doing an orchestration, or simply improvising on solo piano or guitar, point your guide tone lines toward a diminished seventh chord, drop one pitch a half step and see (that is, hear) where it takes you.