interval
D♭ augmented fourth
D♭lowerGupperA46 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
D♭ up a augmented fourth is G: 6 half steps, and 4 letters counting both ends.
Turn it upside down — put the D♭ 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: D♭ to G is a fourth, D♭ to A♭♭ is a fifth. They sound the same and behave differently.
Every interval on D♭
D♭ → E♭♭minor second · dissonanceD♭ → E♭major second · dissonanceD♭ → F♭minor third · imperfect consonanceD♭ → Fmajor third · imperfect consonanceD♭ → G♭perfect fourth · perfect consonanceD♭ → A♭♭diminished fifth · dissonanceD♭ → A♭perfect fifth · perfect consonanceD♭ → B♭♭minor sixth · imperfect consonanceD♭ → B♭major sixth · imperfect consonanceD♭ → C♭minor seventh · dissonanceD♭ → Cmajor seventh · dissonance