The musical interval used in “Black Sabbath” and nicknamed the Devil’s Interval is the tritone.
A tritone spans six semitones, or three whole tones, creating a tense sound midway between an octave’s two notes. In medieval and Renaissance musical theory, it was associated with the Latin phrase diabolus in musica, meaning “the devil in music,” largely because of its harsh, unstable character.
Tony Iommi’s opening riff places this dissonant interval at the center of the song’s ominous mood. The riff moves from a G-based power chord toward C-sharp, producing the tritone that became one of heavy metal’s defining sounds. The song was recorded in 1969 and released on Black Sabbath’s debut album in 1970.
A common mix-up is calling the interval a “devil chord.” Tritone is the interval itself; it can appear melodically or harmonically, and it is not a chord name. It also predates heavy metal by centuries and appears in classical, jazz, blues, and film music.