interval
A♭ augmented fourth
A♭lowerDupperA46 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♭ → B♭major second · dissonanceA♭ → C♭minor third · imperfect consonanceA♭ → Cmajor third · imperfect consonanceA♭ → D♭perfect fourth · perfect consonanceA♭ → E♭♭diminished fifth · dissonanceA♭ → E♭perfect fifth · perfect consonanceA♭ → F♭minor sixth · imperfect consonanceA♭ → Fmajor sixth · imperfect consonanceA♭ → G♭minor seventh · dissonanceA♭ → Gmajor seventh · dissonance