The three diminished chords: blues, jazz and classical

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!

The blues diminished chord

The diminished chord with its root on the tonic of the key is an important sound in the blues. It probably emerges out of the same just intonation intervals as the blues scales and blue notes. In the key of C, the blues diminished chord is C°7. That’s easy enough to remember.

You can hear the blues diminished chord throughout “Kind Hearted Woman Blues” by Robert Johnson. The tune is in A, and Johnson alternates between A7 and A°7 repeatedly.

The blues diminished chord has an inversion whose root is on bIII. In C, that’s Eb°7 (or D#°7, same thing). This chord is part of a family of widely used riffs. In C blues, you use Eb°7 as part of a walkup or walkdown between different inversions of C7.

The inversion of the tonic diminished whose root is on #IV is yet another crucial blues chord. In C, that’s F#°7. You often hear it in the sixth bar of twelve bar blues in between IV7 and I7. In C, that’s F7, F#°7, C7. In intros and endings, you also hear IV7 to #IV°7 to V7. In C, that’s F7, F#°7, G7.

Gospel and jazz songs use #IV°7 too. Check out “I Wish I Knew How It Would Feel To Be Free“, as made famous by Nina Simone. The tune is in B-flat. In bar 14, it does a nifty variant on the blues progression, moving from Gm to E°7 to F7.

https://www.youtube.com/watch?v=HDqmJEWOJRI

If you are soloing, any of the blues scales will harmonize delightfully well with the blues diminished chord. And arpeggiating the blues diminished chord makes a nice blues lick too. The diminished seventh chord has two tritones in it – in C°7 they’re between C and F-sharp, and between E-flat and A. There’s a lot of blues flavor in there.

The classical diminished chord

Classical music mostly uses diminished chords whose roots are a half step below the tonic. In C, that’s B°7.

This chord acts as a minor key dominant, doing the same job as V7. So in C minor, B°7 serves the same function as G7. Notice that B°7 and G7 share three notes in common: B, D, and F. If you raise the G in G7 by a half step, you get Ab°7, which is an inversion of B°7.

For an iconic usage of the classical diminished chord, listen to Bach’s Toccata and Fugue in D minor. In the recording below, there’s a dramatic C#°7 at 0:22.

The jazz diminished chord

Many jazz standards, such as rhythm changes tunes, use the diminished chord whose root is a half step above the tonic. In the key of C, that’s C#°7.

The typical use case is to put this chord in between the tonic and the ii chord. So in C, you would go Cmaj7, C#°7, Dm7.

If you think of Dm as the temporary key center, then C#°7 is a rootless voicing of A7(b9). This means that the jazz diminished chord is really just the classical diminished chord, but acting on ii rather than I.

You can hear the jazz diminished chord in “It’s Only A Paper Moon”. This recording is in B-flat, and the melody begins with Bb6, B°7, Cm7, F7.

So what scale goes with this chord? The thing to keep in mind is that the jazz diminished chord is standing in for V7/ii. If you’re in C and the ii chord is D minor, you’re thinking of D minor as temporary home base, so you can play D harmonic minor over the C#°7 chord (because it’s effectively A7). A more modern jazz musician would use the C-sharp whole-half diminished scale in this context, but get your feet under you with harmonic minor first.

I made a track that lets you compare the three diminished chords side by side:

Here’s a (rhythmically simplified) chart:

My nicknames for these chords are gross oversimplifications and should not be taken as any kind of rigorous classification scheme. You can hear any of the three diminished chords in any style of music, and there are many more use cases for them. But my scheme makes for a good starting point.

Update: a joyless pedant on the Facebook Music Theory group took me to task for spelling C°7 wrong. It’s technically supposed to be C, E-flat, G-flat, B-flat-flat. I intentionally wrote that last note as an A, for several reasons. One is just that double flats are off-putting and confusing to beginners. Two, in the blues, the tonic diminished chord is a variant on IV7, so in C, A is really a more “correct” spelling than B-flat-flat. Three, I’m a jazz theorist, and in jazz theory, enharmonic equivalence rules.