The ♯9 of B♭ is C♯
The ♯9 (sharp nine, written #9) of B♭ (B flat, Bb) is C♯ (C sharp): an augmented ninth above the root.
How to find it
A 9th is a 2nd an octave up, so it takes the 2nd's letter: B, C.
Then the sound: an augmented ninth is 15 semitones above B♭, and the C that is 15 semitones up is C♯.
That is why it is C♯ and not D♭: D♭ is the same key on the piano, but from B♭ it would be a 10th, not a 9th.
Where you meet it
The ♯9 is the tension of the 7♯9 chord. It sounds like the minor 3rd, but it is a raised 9th and is spelled with the 9th's letter.
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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