interval
C♯ major seventh
C♯lowerB♯upperM711 half steps
Eleven half steps: a half step short of the octave, which is why it leans on it.
On the keyboard
What is true of it
C♯ up a major seventh is B♯: 11 half steps, and 7 letters counting both ends.
Turn it upside down — put the C♯ on top — and it becomes a minor second. The two numbers always add to nine, and the two sizes to twelve.
It is a dissonance.
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♯ → A♯major sixth · imperfect consonanceC♯ → Bminor seventh · dissonance