Scale degrees · B♭ major
The ♭13 of B♭ is G♭
The ♭13 (flat thirteen, written b13) of B♭ (B flat, Bb) is G♭ (G flat): a minor thirteenth above the root.
B♭ — the root, 1G♭ — the ♭13
How to find it
A 13th is a 6th an octave up, so it takes the 6th's letter: B, C, D, E, F, G.
Then the sound: a minor thirteenth is 20 semitones above B♭, and the G that is 20 semitones up is G♭.
That is why it is G♭ and not F♯: F♯ is the same key on the piano, but from B♭ it would be a 12th, not a 13th.
Where you meet it
The ♭13 is an altered tension of the dominant chord, one of the notes of the altered scale.
In B♭7♭13:
B♭ · D · F · A♭ · G♭
The ♭13 in every key
| Key | The ♭13 |
|---|---|
| C | A♭ |
| G | E♭ |
| D | B♭ |
| A | F |
| E | C |
| B | G |
| F♯ | D |
| D♭ | B𝄫 |
| A♭ | F♭ |
| E♭ | C♭ |
| B♭ | G♭ |
| F | D♭ |
Every degree of B♭
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