interval
G diminished fifth
GlowerD♭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
G up a diminished fifth is D♭: 6 half steps, and 5 letters counting both ends.
Turn it upside down — put the G 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: G to C♯ is a fourth, G to D♭ is a fifth. They sound the same and behave differently.
Every interval on G
G → A♭minor second · dissonanceG → Amajor second · dissonanceG → B♭minor third · imperfect consonanceG → Bmajor third · imperfect consonanceG → Cperfect fourth · perfect consonanceG → C♯augmented fourth · dissonanceG → Dperfect fifth · perfect consonanceG → E♭minor sixth · imperfect consonanceG → Emajor sixth · imperfect consonanceG → Fminor seventh · dissonanceG → F♯major seventh · dissonance