Black Sabbath are an English rock band formed in Birmingham in 1968 by guitarist Tony Iommi, drummer Bill Ward, bassist Geezer Butler, and vocalist Ozzy Osbourne. They are often cited as pioneers of heavy metal music. The band helped define the genre with their first three albums
Black Sabbath,
Paranoid, and
Master of Reality (1971). Throughout the 1980s and 1990s, the band underwent multiple line-up changes, with Iommi being the only constant member throughout its history, though since 1997 it has settled with the original band except for Ward on occasions.