The blurry-sounding guitar line that leads the Beatles’ “Think for Yourself” from 1965’s Rubber Soul? That’s actually no guitar at all.

It’s a bass part, played by Paul McCartney and fed through a device called a fuzz box, which creates a kind of grainy, distorted effect. (There’s a regular bass part, too.)

And as with many Beatles tracks, it broke new ground — “Think for Yourself,” written by George Harrison, marked the first time this device was used on purpose with a bass.

‘Don’t Worry’ and ‘Zip-a-Dee-Doo-Dah’

Four years prior in 1961, a similar sound was created in Marty Robbins’ “Don’t Worry,” a huge No. 1 hit on the country chart. But in that case, the fuzzy bass sound was essentially made on accident, the result of some kind of technical malfunction that Robbins ultimately decided to keep on the track.

A year after that, producer Phil Spector helped Bob B. Soxx & the Blue Jeans achieve something similar on their cover of “Zip-a-Dee-Doo-Dah.” According to Harrison, who worked with Spector on a number of occasions after the Beatles split up, it was a makeshift thing.

“When Phil Spector was making ‘Zip-A-Dee-Doo-Dah,’ the engineer who’s set up the track overloaded the microphone on the guitar player and it became very distorted,” he said for Anthology. “Phil Spector said, ‘Leave it like that, it’s great.’

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By the time the Beatles were working on “Think for Yourself,” an actual piece of commercial equipment was available.

“Some years later everyone started to try to copy that sound and so they invented the fuzz box,” Harrison continued. “We had one and tried the bass through it and it sounded really good.”

Listen to the Beatles’ ‘Think for Yourself’

The Beatles were never the type to shy away from trying something unorthodox.

“The first time we heard a tape go backwards by mistake we went ‘Oh what’s that?’ and we wanted to put that on our record,” McCartney said to BBC Radio 1 in 2023, “whereas other people would just go ‘Oh come on get the tape on the right way and let’s get on with it.’