interval
D diminished fifth
DlowerA♭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 → Emajor second · dissonanceD → Fminor third · imperfect consonanceD → F♯major third · imperfect consonanceD → Gperfect fourth · perfect consonanceD → G♯augmented fourth · dissonanceD → Aperfect fifth · perfect consonanceD → B♭minor sixth · imperfect consonanceD → Bmajor sixth · imperfect consonanceD → Cminor seventh · dissonanceD → C♯major seventh · dissonance