Why do different joints allow different movements?

A joint is a place where bones meet. An elbow-like hinge mainly bends and straightens in one plane. A shoulder-like ball-and-socket joint can move in more directions. The model keeps the bones joined while comparing permitted movement. A joint’s geometry constrains how connected bones can move. Hinge motion is mainly bending and straightening, while a ball-and-socket connection allows several movement directions. Real joints also depend on cartilage, capsules, ligaments and muscle control. The model isolates geometry; it does not imply unrestricted movement or prescribe a safe angle.