In music, what is the interval between C and F sharp?

In music, the interval between C and F sharp is an augmented fourth.

Counting letter names from C gives C–D–E–F, so the interval is some kind of fourth. A perfect fourth from C is F, spanning five semitones. Raising F to F sharp widens that interval by one semitone, making C–F sharp an augmented fourth, abbreviated A4.

C–F sharp is also a tritone because it spans six semitones, or three whole tones. The word “tritone” describes its size, while “augmented fourth” describes its diatonic spelling. That distinction matters: C to G flat has the same sounding pitch distance on equal-tempered instruments but is normally spelled a diminished fifth.

Historically, the tritone acquired an ominous reputation in some medieval accounts and was nicknamed diabolus in musica, or “devil in music,” though the phrase’s history is more complicated than the popular legend suggests. In modern harmony, the interval is routine: it appears in dominant seventh chords, where the third and seventh create a tritone that strongly seeks resolution.