The ♭7 of B♭ is A♭
The ♭7 (flat seven, written b7) of B♭ (B flat, Bb) is A♭ (A flat): a minor 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 minor seventh is 10 semitones above B♭, and the A that is 10 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 dominant chord and of the minor seventh chord, and the note that turns the major scale into Mixolydian.
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♭ |
Every degree of B♭
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