Although Black Sabbath went through many line-ups and stylistic changes, their core sound focuses on ominous lyrics and doomy music, often making use of the musical tritone, also called the "devil's interval". While their Ozzy-era albums such as Sabbath Bloody Sabbath (1973) had slight compositional similarities to the progressive rock genre that was growing in popularity at the time, standing in stark contrast to popular music of the early 1970s Black Sabbath's dark sound was dismissed by rock critics of the era. Much like many of their early heavy metal contemporaries, the band received virtually no airplay on rock radio.
As the band's primary songwriter, Tony Iommi wrote the majority of Black Sabbath's music, while Osbourne would write vocal melodies, and bassist Geezer Butler would write lyrics. The process was sometimes frustrating for Iommi, who often felt pressured to come up with new material: "If I didn't come up with anything, nobody would do anything." On Iommi's influence, Osbourne later said:
Black Sabbath never used to write a structured song. There'd be a long intro that would go into a jazz piece, then go all folky... and it worked. Tony Iommi—and I have said this a zillion times—should be up there with the greats. He can pick up a guitar, play a riff, and you say, 'He's gotta be out now, he can't top that.' Then you come back, and I bet you a billion dollars, he'd come up with a riff that'd knock your fucking socks off.
Beginning with their third album, Master of Reality (1971), Black Sabbath began to feature tuned-down guitars. In 1965, before forming Black Sabbath, guitarist Tony Iommi suffered an accident while working in a sheet metal factory, losing the tips of two fingers on his right hand. Iommi almost gave up music, but was urged by the factory manager to listen to Django Reinhardt, a jazz guitarist who lost the use of two fingers in a fire. Inspired by Reinhardt, Iommi created two thimbles made of plastic and leather to cap off his missing fingertips. The guitarist began using lighter strings, and detuning his guitar, to better grip the strings with his prosthesis. Early in the band's history Iommi experimented with different dropped tunings, including C♯ tuning, or 3 semitones down, before settling on E♭/D♯ tuning, or a half-step down from standard tuning.