Joints

A joint is any point where two or more bones meet. Joints are what turn a rigid skeleton into a body that can bend, twist, and reach — without them, the skeleton would be a single fused shell. Different joints allow very different amounts and types of motion, matched to the job each one does.

A ball-and-socket joint, like the shoulder or hip, lets one bone rotate freely in almost any direction — essential for a joint that needs a wide range of motion. A hinge joint, like the elbow or knee, allows movement mainly in one plane, like a door swinging on its hinges — a simpler, more stable design for a joint that needs strength more than flexibility. Some joints, like those between the bones of the skull, are essentially fixed and allow almost no movement at all, which is exactly what you want for a structure meant to protect the brain.