Here's why:
* Monkeys are generally smaller primates with tails. They belong to the infraorder Simiiformes, which is further divided into New World monkeys and Old World monkeys.
* Apes are tailless primates belonging to the family Hominidae. This family includes humans, chimpanzees, gorillas, orangutans, and bonobos.
Therefore, chimpanzees, being tailless and belonging to the family Hominidae, are classified as apes, not monkeys.