It didn’t take long for Paul to point out that the chords John was playing weren’t proper guitar chords. Paul showed John the guitar chords he knew, including that magical B7 recently learned from a distant stranger. When sitting opposite one another, John and Paul’s guitars went the same way, the consequence of Paul being left-handed. He encouraged John to study his fingers, to jettison the banjo notes and learn proper guitar chords.
— The Beatles: All these years - extended special edition by Mark Lewisohn.