John, Paul, George, and Ringo. The Beatles were a tight-knit gang of four - christened the Fab Four in 1963 after the press picked up the term from the sleeve notes for their second album. When it came to the band's distinctive sound, Paul is now in no doubt that The Beatles owed the most to their record producer Sir George Martin, who has died, aged 90. "If anyone earned the title of 'the fifth Beatle,' it was George," he wrote.

Source: BBC