On the tracks "Children of the Grave", "Lord of This World", and "Into the Void", Iommi downtuned his guitar 1 steps in an effort to reduce string tension, thus making the guitar less painful for him to play. This pain was the result of a factory accident years earlier in which he had the tips of two of his fingers severed. The downtuning also helped the guitarist produce what he called a "bigger, heavier sound". Geezer Butler also downtuned his bass guitar to match Iommi. "It helped with the sound, too", Butler explained to "Guitar for the Practicing Musician" in 1994. "Then it got to the point where we tuned even lower to make it easier vocal-wise. But Ozzy (Osbourne) would then sing higher so it sort of defeated the object." In the 2013 biography of the band "Black Sabbath: Symptom of the Universe", Mick Wall writes that "the Sabbath sound took a plunge into even greater darkness. Bereft even of reverb, leaving their sound as dry as old bones dug up from some desert burial plot, the finished music's brutish force would so alarm the critics they would punish Sabbath in print for being blatantly thuggish, purposefully mindless, creepy, and obnoxious. Twenty years later groups like Smashing Pumpkins, Soundgarden, and, particularly, Nirvana, would excavate the same heaving lung sound ... And be rewarded with critical garlands." In his autobiography "I Am Ozzy," vocalist Osbourne states that he cannot remember much about recording "Master of Reality" "apart from the fact that Tony detuned his guitar to make it easier to play, Geezer wrote 'Sweet Leaf' about all the dope we'd been smoking, and 'Children of the Grave' was the most kick-ass song we'd ever recorded."