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