interval
C♯ perfect fourth
C♯lowerF♯upperP45 half steps
Five half steps. Turned upside down it becomes a perfect fifth.
On the keyboard
What is true of it
C♯ up a perfect fourth is F♯: 5 half steps, and 4 letters counting both ends.
Turn it upside down — put the C♯ on top — and it becomes a perfect fifth. The two numbers always add to nine, and the two sizes to twelve.
It is a perfect consonance.
Every interval on C♯
C♯ → Dminor second · dissonanceC♯ → D♯major second · dissonanceC♯ → Eminor third · imperfect consonanceC♯ → E♯major third · imperfect consonanceC♯ → F𝄪augmented fourth · dissonanceC♯ → Gdiminished fifth · dissonanceC♯ → G♯perfect fifth · perfect consonanceC♯ → Aminor sixth · imperfect consonanceC♯ → A♯major sixth · imperfect consonanceC♯ → Bminor seventh · dissonanceC♯ → B♯major seventh · dissonance