Diminished seventh chords are strange creatures: a cliche for Dracula’s castle, but also a cornerstone of the blues. They are also difficult to understand. The good news is that in any given key, there are only three possible diminished seventh chords: the one whose root is the tonic of the key, the one whose root is a half step below the tonic, and the one whose root is a half step above the tonic. In the key of C, these are C°7, B°7, and C#°7 respectively. There are no other possible diminished seventh chords!
- C°7 is made of the same notes as D#°7/Eb°7, F#°7/Gb°7, and A°7.
- B°7 is made of the same notes as D°7, F°7, and G#°7/Ab°7.
- C#°7 (or Db°7) is made of the same notes as E°7, G°7, and A#°7/Bb°7.
There are many arcane and unmemorable names for these three chords. I like to think of them as the blues diminished chord, the classical diminished chord, and the jazz diminished chord. Here they are on the chromatic circle against a C root.
You can see how simple and symmetrical these chords are: just stacks of minor thirds. But this simple structure hides a lot of harmonic intrigue. Let’s dig in! Continue reading “The three diminished chords: blues, jazz and classical”