interval
B diminished fifth
BlowerFupperd56 half steps
A perfect fifth narrowed by a half step. The interval inside every diminished triad.
On the keyboard
What is true of it
B up a diminished fifth is F: 6 half steps, and 5 letters counting both ends.
Turn it upside down — put the B on top — and it becomes a augmented fourth. 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 → E♯augmented fourth · dissonanceB → F♯perfect fifth · perfect consonanceB → Gminor sixth · imperfect consonanceB → G♯major sixth · imperfect consonanceB → Aminor seventh · dissonanceB → A♯major seventh · dissonance