interval
C augmented fourth
ClowerF♯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 → D♭minor second · dissonanceC → Dmajor second · dissonanceC → E♭minor third · imperfect consonanceC → Emajor third · imperfect consonanceC → Fperfect fourth · perfect consonanceC → G♭diminished fifth · dissonanceC → Gperfect fifth · perfect consonanceC → A♭minor sixth · imperfect consonanceC → Amajor sixth · imperfect consonanceC → B♭minor seventh · dissonanceC → Bmajor seventh · dissonance