interval
B♭ diminished fifth
B♭lowerF♭upperd56 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♭ → C♭minor second · dissonanceB♭ → Cmajor second · dissonanceB♭ → D♭minor third · imperfect consonanceB♭ → Dmajor third · imperfect consonanceB♭ → E♭perfect fourth · perfect consonanceB♭ → Eaugmented fourth · dissonanceB♭ → Fperfect fifth · perfect consonanceB♭ → G♭minor sixth · imperfect consonanceB♭ → Gmajor sixth · imperfect consonanceB♭ → A♭minor seventh · dissonanceB♭ → Amajor seventh · dissonance