The story goes that when Black Sabbath were jamming material in their early days, bassist Geezer Butler was noodling around on the main theme of Mars, The Bringer Of War, from Holst’s The Planets. He played the tonic note (G), the 5th (D), then, crucially, the flattened 5th (Db) that gives Mars its dissonant, menacing feel. With his ears pricking up, the band’s guitarist Tony Iommi then adapted it – G, G an octave up, then that nasty Db in between – thus minting the riff for Black Sabbath. The opening song on their 1970 namesake debut album, the band’s self-titled song would not only define them, but also the crunching sound and doomladen occult atmosphere that legions of metal bands have explored ever since.