interval
C♯ major sixth
C♯lowerA♯upperM69 half steps
Nine half steps. The minor third turned upside down.
On the keyboard
What is true of it
C♯ up a major sixth is A♯: 9 half steps, and 6 letters counting both ends.
Turn it upside down — put the C♯ on top — and it becomes a minor third. The two numbers always add to nine, and the two sizes to twelve.
It is a imperfect consonance.
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♯ → Gdiminished fifth · dissonanceC♯ → G♯perfect fifth · perfect consonanceC♯ → Aminor sixth · imperfect consonanceC♯ → Bminor seventh · dissonanceC♯ → B♯major seventh · dissonance