The Beatles are nicknamed “The Fab Four”, but they were in fact once a 5-piece band. This line up consisted of guitarists John Lennon, Paul McCartney, and George Harrison, bassist Stuart Sutcliffe, and drummer Pete Best.
Sutcliffe was a Beatle from January 1960, until he left the group in June 1961. With his departure, McCartney switched from guitar to bass to fill that vacancy.
Best was a Beatle from August 12, 1960, until he was dismissed from the band on August 16, 1962, when he was replaced by drummer Ringo Starr.
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