interval
F♯ augmented fourth
F♯lowerB♯upperA46 half steps
A perfect fourth widened by a half step. Same sound as the diminished fifth, different letter, different behaviour.
On the keyboard
What is true of it
F♯ up a augmented fourth is B♯: 6 half steps, and 4 letters counting both ends.
Turn it upside down — put the F♯ on top — and it becomes a diminished fifth. The two numbers always add to nine, and the two sizes to twelve.
It is a dissonance.
Six half steps can be written two ways: F♯ to B♯ is a fourth, F♯ to C is a fifth. They sound the same and behave differently.
Every interval on F♯
F♯ → Gminor second · dissonanceF♯ → G♯major second · dissonanceF♯ → Aminor third · imperfect consonanceF♯ → A♯major third · imperfect consonanceF♯ → Bperfect fourth · perfect consonanceF♯ → Cdiminished fifth · dissonanceF♯ → C♯perfect fifth · perfect consonanceF♯ → Dminor sixth · imperfect consonanceF♯ → D♯major sixth · imperfect consonanceF♯ → Eminor seventh · dissonanceF♯ → E♯major seventh · dissonance