The 𝄫7 of E is D♭
The 𝄫7 (double-flat seven, written bb7) of E is D♭ (D flat): a diminished seventh above the root.
How to find it
Count letters first, the root included: E, F, G, A, B, C, D. A 7th lands on D.
Then the sound: a diminished seventh is 9 semitones above E, and the D that is 9 semitones up is D♭.
That is why it is D♭ and not C♯: C♯ is the same key on the piano, but from E 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 E°7:
E · G · B♭ · D♭
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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