When the Grammys take place at 8 p.m. on Sunday, duking it out for Album of the Year will be pop diva Adele, genre-defying icon Beyoncé, an EDM-inflected Justin Bieber, hip-hop star Drake and Sturgill Simpson — a country singer few people outside of Nashville had heard of a year ago.

It’s a motley crew of competition that never would’ve even happened had the year 1991 not changed music forever.

Back then, as now, the Grammys largely reflected the albums that had dominated the Billboard charts: Quincy Jones, Mariah Carey — and, in a show of just how bleak times were — Phil Collins, Wilson Phillips and MC Hammer.

Never forget: MC Hammer’s “Please Hammer Don’t Hurt ’Em” was nominated for Album of the Year.

But as Derek Thompson explains in his new book “Hit Makers,” 1991 was the year Billboard made a deal with Nielsen SoundScan to record albums sales at point of purchase from cash registers, rather than relying on information provided by retailers. Traditionally, it suited both retailers and radio stations to doctor figures: Stores wanted to hype the albums that were plentiful in stock, while radio programmers were more than happy to receive gifts from labels needing to push their biggest (read: costliest) acts.

With the changing rules, the album charts became a truer reflection of what Americans actually listened to — and, it turned out, we were often cooler than stores gave us credit for. People (lots of them!) were listening to hip-hop, which had long been rubbished by the industry as a fad. “I had conversations with people at labels and at Billboard who said that hip-hop was being suppressed before the rule changes,” Thompson told The Post.

Thanks to SoundScan, it was no longer possible to keep this new genre down. Albums from N.W.A (“Niggaz4Life”), Ice Cube (“The Predator”) and Cypress Hill (“Black Sunday”) all hit No. 1 in the early ’90s.

Rock music also struck a different tone, with heavier bands such as Metallica finding mainstream success by the end of 1991. But it was the sound of Nirvana that caused the biggest rupture to the guitar-based status quo.

The Seattle grunge trio’s second album, “Nevermind,” came out relatively quietly in September of 1991 but gradually rose to the top of the charts, displacing none other than Michael Jackson — an unthinkable achievement in what was previously a rigged game.

(And to think, the recent ‘90s fashion revival could’ve been centered around Hammer pants had Kurt Cobain and his ripped jeans and flannels never hit it big.)

Country-music artists also suddenly had a place at the table. For more than half of 1992, albums by Garth Brooks or Billy Ray Cyrus held the No. 1 position.

There has been one major, if unintentional, drawback, though. While SoundScan saved us from the domination of industry graft, the new meritocracy ushered in an era of repetition.

“Without the music labels manipulating the charts, Billboard was a more perfect mirror of American tastes, and the reflection in the mirror said: ‘Just play the hits!’ ” writes Thompson. As a result, the top 10 longest-running Billboard No. 1 singles have happened during the last 26 years.

Mariah Carey and Boyz II Men’s “One Sweet Day” still holds the record, with a mind-numbing 16 weeks at the top slot in 1995-96. You can also blame the new system for such inescapable aberrations as Los Del Río’s “Macarena” (14 weeks in 1996) and the Black Eyed Peas’ “I Gotta Feeling” (14 weeks in 2009).

So, in a way, one kind of tyranny was replaced with another. But at least this is one that music fans inflicted on themselves.

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