interval
C♯ augmented fourth
C♯lowerF𝄪upperA46 half steps
A perfect fourth widened by a half step. Same sound as the diminished fifth, different letter, different behaviour.
On the keyboard
What is true of it
C♯ up a augmented fourth is F𝄪: 6 half steps, and 4 letters counting both ends.
Turn it upside down — put the C♯ on top — and it becomes a diminished fifth. 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♯ → Gdiminished fifth · dissonanceC♯ → G♯perfect fifth · perfect consonanceC♯ → Aminor sixth · imperfect consonanceC♯ → A♯major sixth · imperfect consonanceC♯ → Bminor seventh · dissonanceC♯ → B♯major seventh · dissonance