Others assume the form of hinge joints, in which convexities or prominences on one bone fit into depressions or grooves in its fellow. In this form of articulation the movement of the joint, like that of a door, is only to and fro or in the direction of flexion and extension. A good example of this class of articulation will be found in the tibio-tarsal or true hock joint (fig. 361), the elbow-joint, etc.