interval
C♯ diminished fifth
C♯lowerGupperd56 half steps
A perfect fifth narrowed by a half step. The interval inside every diminished triad.
On the keyboard
What is true of it
C♯ up a diminished fifth is G: 6 half steps, and 5 letters counting both ends.
Turn it upside down — put the C♯ 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: C♯ to F𝄪 is a fourth, C♯ to G is a fifth. They sound the same and behave differently.
Every interval on C♯
C♯ → Dminor second · dissonanceC♯ → D♯major second · dissonanceC♯ → Eminor third · imperfect consonanceC♯ → E♯major third · imperfect consonanceC♯ → F♯perfect fourth · perfect consonanceC♯ → F𝄪augmented fourth · dissonanceC♯ → G♯perfect fifth · perfect consonanceC♯ → Aminor sixth · imperfect consonanceC♯ → A♯major sixth · imperfect consonanceC♯ → Bminor seventh · dissonanceC♯ → B♯major seventh · dissonance