Scale degrees · B♭ major
The ♭9 of B♭ is C♭
The ♭9 (flat nine, written b9) of B♭ (B flat, Bb) is C♭ (C flat): a minor ninth above the root.
B♭ — the root, 1C♭ — the ♭9
How to find it
A 9th is a 2nd an octave up, so it takes the 2nd's letter: B, C.
Then the sound: a minor ninth is 13 semitones above B♭, and the C that is 13 semitones up is C♭.
That is why it is C♭ and not B: B is the same key on the piano, but from B♭ 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 B♭7♭9:
B♭ · D · F · A♭ · C♭
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♭ |
Every degree of B♭
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