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