Polydactyly is the congenital malformation of having extra digits - either fingers or toes. It is associated with a genetic mutation. The condition is surprisingly common - 1 in 500 infants show some form of polydactyly, however if the extra digit is non-functional it is usually removed shortly after birth (often the “finger” is just tissue with no bones or joints). The most common case is an additional pinky, then thumb, and most rarely seen is a central digit.
Anne Boleyn, second wife of King Henry XIII and mother of Queen Elizabeth, had six fingers on her right hand. This, among other things, led many to believe that she was a witch, as polydactyly has been historically associated with witchcraft and other evils.