Black Sabbath kicked loose rock that's still setting off avalanches of creativity across the musical landscape. Their phenomenal first four albums -- Black Sabbath, Paranoid, Master of Reality, and Vol. 4 -- remain the clearest articulations of heavy metal vocabulary to date. Bassist Geezer Butler, the man responsible for the lion's share of Sabbath's early hits, focused his lyrics on social ills, self-destructive behavior, fantasy and the occult, setting forth the primary concerns which continue to preoccupy metal bands from velocity addicts Venom and Napalm Death to sludge fiends such as Monster Magnet. Lead guitarist Tony Iommi's loose tunings revolutionized the craft of electric guitar by uncovering unsuspected fuzzy depths. And, of course, Ozzy Osbourne's manic live performances introduced an element of pure theatricality that remains central to metal's identity. Beginning with '75's Sabotage, Sabbath's credibility has been constantly imperiled by failed experiments in prog-ish rock. With the exception of the Ronnie James Dio albums of the early '80s (Heaven and Hell and Mob Rules), the moments when Sabbath play up to their ability have been few and far between. Perhaps aware of this themselves, they convinced Ozzy to return to the fold in the late '90s for a series of penitential performances focusing on their glorious early material.