interval
A♯ perfect fourth
A♯lowerD♯upperP45 half steps
Five half steps. Turned upside down it becomes a perfect fifth.
On the keyboard
What is true of it
A♯ up a perfect fourth is D♯: 5 half steps, and 4 letters counting both ends.
Turn it upside down — put the A♯ 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 A♯
A♯ → Bminor second · dissonanceA♯ → B♯major second · dissonanceA♯ → C♯minor third · imperfect consonanceA♯ → C𝄪major third · imperfect consonanceA♯ → D𝄪augmented fourth · dissonanceA♯ → Ediminished fifth · dissonanceA♯ → E♯perfect fifth · perfect consonanceA♯ → F♯minor sixth · imperfect consonanceA♯ → F𝄪major sixth · imperfect consonanceA♯ → G♯minor seventh · dissonanceA♯ → G𝄪major seventh · dissonance