Why does Cdim7 contain an A instead of a Bb?

5 Answers. 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.

1. Why does C7 have a Bb?
2. What key is Cdim7 in?

Why does C7 have a Bb?

The real short answer is that the C7 chord doesn't go in the key of C. It would fit in the key of F. If you're in the key of C, and you add a seventh to the C chord, you get a Cmaj7, which has the B natural.

What key is Cdim7 in?

Explanation: The Cdim7 is a four-note chord. Cdim7 can also be written as C°7. Theory: The Cdim7 is a diminished chord extended with a minor third.