You'll never guess...
SPOILER ALERT!
...did you guess the 5 string banjo? Yes it's true! His Dad was a professional musician, played for cruise ships and was always away. His mom played the 5 string banjo, but didn't want little Johnny to follow in his Daddy's footsteps, so she forbade him. She wouldn't let him play the guitar. He lived with his Aunt, who was also pretty strict about that. But one day his Mom showed him a few things on the banjo. He remembered everything he learned, so when he finally got together with his new buds Paul and George, he took a string off the guitar and tuned it like the banjo.
True story!