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Master of Reality is the third studio album by English rock band Black Sabbath, released in July 1971. It is widely regarded as the foundation of doom metal, stoner rock, and sludge metal. It was certified double platinum after having sold over 2 million copies. Master of Reality was Black Sabbath's first and only top 10 album in the US until 13, forty-two years later.

Master of Reality was recorded at Island Studios, in London, during February and April 1971. The album was produced by Rodger Bain, who had also produced Black Sabbath's previous two albums; this was to be his final collaboration with the band.

On the tracks "Children of the Grave", "Lord of This World", and "Into the Void", guitarist Tony Iommi downtuned his guitar 1 ¹⁄₂ steps to produce what he called a "bigger, heavier sound". This also reduced string tension, thus making the guitar less painful for him to play; Iommi had two of his fingers partially severed in a factory accident years earlier.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 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 Ozzy 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."

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