All the descendants of an ancestor make up a monophyletic group, or clade,
and are more closely related to each other than other organisms.
Birds and crocodiles compose a clade because they share a recent common ancestor (A).
Lizards and crocodiles do not compose a clade; their common ancestor (B) is also shared by birds.
Birds, crocodiles, and lizards do compose a larger clade.
An even larger clade includes all 4 organisms: terrestrial vertebrates with 4 limbs (tetrapods).