Heavy music was around before Black Sabbath issued Master of Reality, their third album, on July 21, 1971. But the guidelines for how heavy music sounded after it came out were pretty much written on the record's eight tracks.

A few months after Sabbath's second LP, Paranoid, the four members holed up once again in a London studio with Rodger Bain – the producer who had worked on both of the group's 1970 albums, Paranoid and the self-titled debut. For two months, from February through April, Black Sabbath hammered out a handful of new songs that were heavier, denser and more forceful than anything found on the first two albums.

And much of it pretty much happened by accident.

The thick, airless music that Master of Reality is so celebrated for was a byproduct of Tony Iommi, who was missing parts of two fingers, down-tuning his guitar so it would be easier for him to play. Bassist Geezer Butler followed suit, and soon the entire project took on a sludgier, doom-invoking tone.

The results of these lower-tuned instruments became a launching point for generations of musicians. The deep, nearly suffocating sounds on the album – in addition to the usual guitar, bass and drums, Iommi plays synths, piano and flute on a pair of tracks – influenced everything from heavy metal to grunge to stoner rock over the years. The LP's best tracks (especially "Sweet Leaf" and "Children of the Grave") serve as the cornerstones to both modern heavy rock and Sabbath's career.

Listen to Black Sabbath's 'Sweet Leaf'

And like this accidental stumble into this new musical territory, "Sweet Leaf" – the record's opening track – also found its signature moment by chance. In his 2011 autobiography Iron Man: My Journey Through Heaven and Hell With Black Sabbath, Iommi said his cough that starts the song and album was the result of a joint handed to him by singer Ozzy Osbourne while he was recording. Appropriately, the band was high when it made "Sweet Leaf" and most of Master of Reality.

That too is part of its legacy. Even though drug use at the time was pretty much a given thing for rock artists, no other album – certainly no other album that matters as much – captures its effects so naturally. Listening to Master of Reality is like wading though a thick fog of marijuana smoke and clawing away at curtains of clarity. Everything every stoner-rock band you can name has done starts right here.

Not that anyone realized it at the time. Contemporary critics were tough on the record, just like they were on Master of Reality's predecessors. It took years before the music press came around to Black Sabbath. But the group was picking up more and more fans with each release. Their popularity soared in the U.S., where Master of Reality was their highest-charting album, at No. 8, until 13 came along in 2013 and debuted at No. 1. In their native U.K., the album peaked at No. 5, a slight dip from Paranoid's No. 1 showing 10 months earlier.

After the album's release, Black Sabbath tempered their recording schedule (their first three albums were released during a hectic 17-month period), and Vol. 4 didn't arrive until more than a year later in September 1972. But by then, things were starting to derail due to the band's increasing substance-abuse problems. They reached their peak on Master of Reality. Its glorious, blurry-eyed heaviness still resonates today.

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