Many people think that the single distinguishing characteristic of New World monkeys as opposed to Old World monkeys is their prehensile tail. Technically speaking, the characteristic between the two isn't their tails it's their noses. New World monkeys have flatter noses than their Old World counterparts.

When speaking of examples of prehensility, most people point to the New World monkey's tail as the definitive example of a prehensile body part completely ignoring the opposable thumb on their own hand.