interval
A augmented fourth
AlowerD♯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 → B♭minor second · dissonanceA → Bmajor second · dissonanceA → Cminor third · imperfect consonanceA → C♯major third · imperfect consonanceA → Dperfect fourth · perfect consonanceA → E♭diminished fifth · dissonanceA → Eperfect fifth · perfect consonanceA → Fminor sixth · imperfect consonanceA → F♯major sixth · imperfect consonanceA → Gminor seventh · dissonanceA → G♯major seventh · dissonance