Why does Cdim7 contain an A instead of a Bb?

Question

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?

Answer 1

It's a B double flat, or Bbb, not an A. This is an enharmonic spelling to stay consistent with the chordal notes because C chords don't contain an A.

Dim 7s are 3 minor third intervals stacked on one another.

So you have a diminished triad, and a diminished 7th.

C Eb Gb Bb, is a diminished triad with a minor seventh. This is called a half diminished chord or minor 7 flat 5 depending if you're in classical or jazz circles.