interval
E♭ augmented fourth
E♭lowerAupperA46 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
E♭ up a augmented fourth is A: 6 half steps, and 4 letters counting both ends.
Turn it upside down — put the E♭ 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: E♭ to A is a fourth, E♭ to B♭♭ is a fifth. They sound the same and behave differently.
Every interval on E♭
E♭ → F♭minor second · dissonanceE♭ → Fmajor second · dissonanceE♭ → G♭minor third · imperfect consonanceE♭ → Gmajor third · imperfect consonanceE♭ → A♭perfect fourth · perfect consonanceE♭ → B♭♭diminished fifth · dissonanceE♭ → B♭perfect fifth · perfect consonanceE♭ → C♭minor sixth · imperfect consonanceE♭ → Cmajor sixth · imperfect consonanceE♭ → D♭minor seventh · dissonanceE♭ → Dmajor seventh · dissonance