The appendicular skeleton

The second major group is the appendicular skeleton — the bones of the arms and legs, plus the two girdles that attach them to the axial skeleton: the shoulder girdle (collarbones and shoulder blades) and the pelvic girdle (the hip bones). Where the axial skeleton is built mainly for protection and support, the appendicular skeleton is built mainly for movement — reaching, walking, grasping, and running.

The arm and leg bones follow a similar pattern to each other: a single long bone near the body (the upper arm bone or thigh bone), then a pair of long bones further out (forearm or lower leg), then a cluster of smaller bones in the hand or foot. This repeating pattern is a good example of how the skeleton reuses a small number of structural "templates" across very different-looking limbs.