CM7 - C, E, G, B

C7 - C, E, G, Bb

Cm7 - C, Eb, G, Bb

Caug7 - C, E, G#, Bb

But...

Cdim7 - C, Eb, Gb, A.

Why Does Cdim7 contain an A and not a Bb?

  • 3
    The other answerers have already addressed the question more than adequately so I won't duplicate them, but I will just point out additionally that there does exist a chord "C Eb Gb Bb" and that it's called the "half diminished" chord, because it has a diminished five but a non-diminished 7th. It's often written as Cø and is also a useful chord (it's really pretty). – Some_Guy Oct 19 at 14:58

C°7 actually includes a B doubleflat.

A major seventh above C is B and a minor seventh above C is B♭. This means that a diminished seventh above C is actually B♭♭, which is enharmonic to A.

But the seventh is not A, because A is just a sixth above C. A is enharmonic to B♭♭, but since we want the seventh above C, it must be B♭♭, not A.

But it gets more interesting: since fully diminished sevenths are just stacked minor thirds, they can be spelled four different ways, with each pitch as root; we say that these chords are enharmonic, just like we say that A is enharmonic to B♭♭.

This then means that C–E♭–G♭–A is a diminished seventh chord, but with A as the root: A–C–E♭–G♭. If E♭ is the root, we'd rather spell it as D♯–F♯–A–C. If G♭ is the root, we'd spell it as F♯–A–C–E♭.

These chords all sound the same as C°7, but they are spelled differently.

  • 'We'd rather spell Ebo as D#o'. Because Ebo would be Eb--Gb--Bbb--Dbb. Good reason! – Tim Oct 20 at 10:55

Other answers have pointed out that it's a B♭♭, not an A. To answer the question "why not a B♭":

The chord in question is the chord of the diminished 7th. As David Bowling and Richard have stated, if the root is C, the diminished 7th is B♭♭.

The chord is also called the "diminished 7th chord". This name and its chord symbol Cdim7 are perhaps confusing. I don't know if this is what you thought, but, just for the record, "Cdim7" does not mean

  • the diminished triad C-E♭-G♭ as indicated by "Cdim"
  • the pitch B♭ as indicated by "7", just as C7 means a C triad plus B♭

It's not a [ diminished triad ] with a seventh. It's a chord of the [ diminished seventh ].

  • 1
    Except it has a diminished triad. It has a m3 and a dim5. Then on top, a dim7. If it was a chord with a dim7 only, it'd sound like a M6. – Tim Oct 20 at 10:48

A Cdim7 chord is actually spelled C-Eb-Gb-Bbb, where the interval from C to Bbb is called a diminished seventh. The interval from C to A is a major sixth, but sometimes you do see people spell Cdim7 as C-Eb-Gb-A for convenience to avoid the double flat, it is just technically incorrect. This chord is sometimes called a fully-diminished chord, in contrast to a half-diminished chord. A half-diminished C chord (Cmin7b5, C-7b5, or C) is spelled: C-Eb-Gb-Bb. These are often found in ii-V-i progressions in jazz.

  • C-E♭-G♭-A is also the correct spelling of the pitches in Adim7. And it's OK to put the C (or indeed E♭ or G♭) in the bass. – Rosie F Oct 19 at 13:24
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    @RosieF -- not sure I understand your point; Adim7 and Cdim7 are enharmonically spelled the same way, but are not the same chords. – David Bowling Oct 19 at 13:28
  • Indeed. I just didn't want anyone reading your answer to think the spelling with an A is as you say "technically incorrect". A composer may write an A and yet be technically correct. It's just that the technically correct chord symbol would then be Adim7. When it comes to deciding how to spell a dim7 chord, let voice leading be your guide. And yes, perhaps simplify for convenience if you feel that's right. The chord symbol comes later. The music determines the chord symbol, not the other way around. The composer needn't let the symbol determine the spelling. – Rosie F Oct 19 at 13:45
  • 2
    @RosieF -- it is a technically incorrect way to spell Cdim7; it is only a correct spelling for Adim7. – David Bowling Oct 19 at 15:34

It actually contains a Bbb (double flat). Often (mis)spelt as A for convenience. You'll also often see it notated as C, Eb, F#, A.

A dim7 chord is a pile of minor 3rds. A minor 3rd up from Gb is Bbb.

Adding a little to the others. Cdim7 cannot contain a Bb note. Bb only produces a minor 7th from the root. To make it a diminished 7th, the space between root and m7 needs to be one semitone smaller. That's the reason it gets called Bbb. It sounds just like A, but since C>A is a M6, that wouldn't be correct if the chord was to be called dim7.

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