Joints are the junctions between two or more bones. Some joints do not normally move, such as those located between the plates of the skull. Other joints allow a large and complex range of motion. The configuration of a joint determines the degree and direction of possible motion. For example, the shoulder joints, which have a ball-and-socket design, allow inward and outward rotation as well as forward, backward, and sideways motion of the arms. Hinge joints of the knees, fingers, and toes allow only bending (flexion) and straightening (extension).
The components of joints provide stability and reduce the risk of damage from constant use. In a joint, the ends of the bones are covered with cartilage—a smooth, tough, resilient, and protective tissue composed of collagen, water, and proteoglycans that reduces friction as joints move. (Collagen is a tough fibrous tissue, and proteoglycans are substances that help provide the cartilage's resilience.) Joints also have a lining (synovial tissue) that encloses them to form the joint capsule. Cells in the synovial tissue produce a small amount of clear fluid (synovial fluid), which provides nourishment to the cartilage and further reduces friction while facilitating movement.