interval
A♯ augmented fourth
A♯lowerD𝄪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
A♯ up a augmented fourth is D𝄪: 6 half steps, and 4 letters counting both ends.
Turn it upside down — put the A♯ 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: A♯ to D𝄪 is a fourth, A♯ to E is a fifth. They sound the same and behave differently.
Every interval on A♯
A♯ → Bminor second · dissonanceA♯ → B♯major second · dissonanceA♯ → C♯minor third · imperfect consonanceA♯ → C𝄪major third · imperfect consonanceA♯ → D♯perfect fourth · perfect consonanceA♯ → 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