interval
D♭ diminished fifth
D♭lowerA♭♭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
D♭ up a diminished fifth is A♭♭: 6 half steps, and 5 letters counting both ends.
Turn it upside down — put the D♭ 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: 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♭ → Gaugmented fourth · dissonanceD♭ → A♭perfect fifth · perfect consonanceD♭ → B♭♭minor sixth · imperfect consonanceD♭ → B♭major sixth · imperfect consonanceD♭ → C♭minor seventh · dissonanceD♭ → Cmajor seventh · dissonance