Geezer Butler started as a guitarist in working-class Birmingham, but when Tony Iommi made clear there was no room for a second guitar, Butler picked up the bass out of necessity. The switch proved defining. His distorted tone, melodic instinct, and tight lock with drummer Bill Ward’s swing created a low-end vocabulary that would mark heavy metal from its first breath. He found his guiding light watching Cream. “I couldn’t believe people could do that with a bass,” Butler recalled in a recent video conversation with Foo Fighter Nate Mendel, marking the 75th anniversary of the Fender Precision bass. “I went, that’s what I want to do—I want to play bass.”