Recording engineer Chris Thomas received a note from producer George Martin on Sept. 9, 1968: "Chris, hope you had a nice holiday," it read. "I'm off on mine now. Make yourself available to the Beatles."

"Paul [McCartney] was the first one to walk in, and he said, 'What are you doing here?'" Thomas recalls in the book The Beatles: Recording Sessions. After the young engineer explained the situation to McCartney, the Beatle responded, "Well, if you wanna produce us, you can produce us. If you don't, we might just tell you to fuck off."

The session was recorded without problems. The song they worked on that day? One of the rawest blasts of primal rock 'n' roll the Beatles ever recorded, "Helter Skelter." An earlier take of the song had been attempted in mid-July. That version clocked in at a whopping 27 minutes. But the group decided to cut a new version to pare it down to a more manageable length.

The Sept. 9 session ran from 7PM to 2:30 the next morning, with take 21 being the magic number. And then it came time to mix the song. There are distinct differences between the mono and stereo mixes of "Helter Skelter" that Beatles obsessives still debate all these years later. (The mono mix, available only on British pressings of the album, wasn't released in the U.S. until 2009.)

McCartney later said that he was inspired by an interview he read with Pete Townshend, who called his newest song for the Who the loudest and most raw thing the band had ever done. McCartney then decided to unleash his inner beast ... and ended up silencing critics who saw him as being too soft. "Helter Skelter" was the Beatles at their heaviest and most raucous – especially at the end of the song, when everything almost falls apart. In the coming years, the track would serve as inspiration to both heavy metal and punk artists feeding on aggression.

The session itself was relatively loose. John Lennon played bass, George Harrison played rhythm guitar, Ringo Starr, naturally, was on drums and McCartney handled vocals and guitar. (And that's Starr who's famously captured on tape at the end of the song screaming "I've got blisters on my fingers!") "While Paul was doing his vocal, George had set fire to an ashtray and was running around the studio with it above his head, doing an Arthur Brown," recalled Thomas. "All in all, a pretty undisciplined session you could say."

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