The 𝄫7 of D♭ is C𝄫
The 𝄫7 (double-flat seven, written bb7) of D♭ (D flat, Db) is C𝄫 (C double flat): a diminished seventh above the root.
How to find it
Count letters first, the root included: D, E, F, G, A, B, C. A 7th lands on C.
Then the sound: a diminished seventh is 9 semitones above D♭, and the C that is 9 semitones up is C𝄫.
That is why it is C𝄫 and not B♭: B♭ is the same key on the piano, but from D♭ it would be a 6th, not a 7th.
Where you meet it
The 𝄫7 is the 7th of the fully diminished chord. On the piano it is the same key as the 6, but a seventh is spelled with a seventh's letter.
In D♭°7:
D♭ · F♭ · A𝄫 · C𝄫
The 𝄫7 in every key
| Key | The 𝄫7 |
|---|---|
| C | B𝄫 |
| G | F♭ |
| D | C♭ |
| A | G♭ |
| E | D♭ |
| B | A♭ |
| F♯ | E♭ |
| D♭ | C𝄫 |
| A♭ | G𝄫 |
| E♭ | D𝄫 |
| B♭ | A𝄫 |
| F | E𝄫 |
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