McCartney Times

Many guitar nerds note that perhaps the most important detail of replicating the Beatles’ tones after their guitars and amps is the type of strings that they used. It is generally believed that Harrison and Lennon used flatwound electric guitar strings in the early years up until late 1965, just after the release of Rubber Soul. After that, from Revolver and beyond, they apparently switched to roundwound strings.

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Source: What guitar strings did the Beatles use? Investigating the flatwound vs roundwound debate | Guitar World