Scale degrees · A♭ major
The ♭9 of A♭ is B𝄫
The ♭9 (flat nine, written b9) of A♭ (A flat, Ab) is B𝄫 (B double flat): a minor ninth above the root.
A♭ — the root, 1B𝄫 — the ♭9
How to find it
A 9th is a 2nd an octave up, so it takes the 2nd's letter: A, B.
Then the sound: a minor ninth is 13 semitones above A♭, and the B that is 13 semitones up is B𝄫.
That is why it is B𝄫 and not A: A is the same key on the piano, but from A♭ it would be an 8th, not a 9th.
Where you meet it
The ♭9 is the first altered tension on a dominant chord. It is the ♭2 an octave up, written as a 9 because it sits above the chord.
In A♭7♭9:
A♭ · C · E♭ · G♭ · B𝄫
The ♭9 in every key
| Key | The ♭9 |
|---|---|
| C | D♭ |
| G | A♭ |
| D | E♭ |
| A | B♭ |
| E | F |
| B | C |
| F♯ | G |
| D♭ | E𝄫 |
| A♭ | B𝄫 |
| E♭ | F♭ |
| B♭ | C♭ |
| F | G♭ |
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