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).