F as a Scale Degree

F is the fourth degree of the C major scale, called Fa in movable-doh solfège, and sits a perfect fourth above the tonic C, an interval historically treated with particular caution in the medieval theory of counterpoint, where the tritone distance between F and the note B, three whole steps away, was nicknamed diabolus in musica, the devil in music, for its harsh, unstable sound in certain melodic and harmonic contexts and was often avoided or carefully resolved by composers working within that tradition.

That reputation belongs to a documented feature of medieval and Renaissance compositional practice rather than to any claim about the note F carrying inherent moral or spiritual danger; later tonal harmony absorbed the tritone as a normal and even essential part of standard chord progressions, particularly in the dominant seventh chord that resolves so characteristically to the tonic in the Western classical and popular repertoire alike.