interval
B augmented fourth
BlowerE♯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
B up a augmented fourth is E♯: 6 half steps, and 4 letters counting both ends.
Turn it upside down — put the B 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: B to E♯ is a fourth, B to F is a fifth. They sound the same and behave differently.
Every interval on B
B → Cminor second · dissonanceB → C♯major second · dissonanceB → Dminor third · imperfect consonanceB → D♯major third · imperfect consonanceB → Eperfect fourth · perfect consonanceB → Fdiminished fifth · dissonanceB → F♯perfect fifth · perfect consonanceB → Gminor sixth · imperfect consonanceB → G♯major sixth · imperfect consonanceB → Aminor seventh · dissonanceB → A♯major seventh · dissonance