10.9Fully diminished seventh

Now:

C°7

is different.

C diminished triad:

C E♭ G♭

Add a diminished seventh above C.

That note is theoretically:

B𝄫

which sounds like A on the piano.

So:

C°7 = C E♭ G♭ B𝄫

On the keyboard:

C E♭ G♭ A

This chord has a special symmetrical structure.

We'll exploit that later.