interval
F♯ perfect fifth
F♯lowerC♯upperP57 half steps
Seven half steps. Stack twelve of them and you land back where you started, give or take a comma.
On the keyboard
What is true of it
F♯ up a perfect fifth is C♯: 7 half steps, and 5 letters counting both ends.
Turn it upside down — put the F♯ on top — and it becomes a perfect fourth. The two numbers always add to nine, and the two sizes to twelve.
It is a perfect consonance.
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♯ → B♯augmented fourth · dissonanceF♯ → Cdiminished fifth · dissonanceF♯ → Dminor sixth · imperfect consonanceF♯ → D♯major sixth · imperfect consonanceF♯ → Eminor seventh · dissonanceF♯ → E♯major seventh · dissonance