The ♯4 of F♯ is B♯
The ♯4 (sharp four, written #4) of F♯ (F sharp, F#) is B♯ (B sharp): an augmented fourth above the root.
How to find it
Count letters first, the root included: F, G, A, B. A 4th lands on B.
Then the sound: an augmented fourth is 6 semitones above F♯, and the B that is 6 semitones up is B♯.
That is why it is B♯ and not C: C is the same key on the piano, but from F♯ it would be a 5th, not a 4th.
Where you meet it
The ♯4 is the Lydian note, a tritone above the root. On a chord symbol it is usually written ♯11.
In the scale F♯ Lydian:
F♯ · G♯ · A♯ · B♯ · C♯ · D♯ · E♯
The ♯4 in every key
| Key | The ♯4 |
|---|---|
| C | F♯ |
| G | C♯ |
| D | G♯ |
| A | D♯ |
| E | A♯ |
| B | E♯ |
| F♯ | B♯ |
| D♭ | G |
| A♭ | D |
| E♭ | A |
| B♭ | E |
| F | B |
Every degree of F♯
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