Tritone

Both songs above contain tritones in prominent places: Black Sabbath uses them to create the main riff and Camille Saint-Saëns uses it in the violin solo.

The tritone is a musical interval that spans three whole tones. The tritone, sometimes known as the “Diabolus in Musica” or “The Devils Interval”, is the same as an augmented fourth, which is the equivalent to a diminished fifth.

Find out more here and here.