The temporomanibular joint is the joint of the jaw and is frequently referred to as TMJ. There are two TMJs, one on either side, working in unison. The name is derived from the two bones which form the joint: the upper temporal bone which is part of the cranium (skull), and the the lower jaw bone called the mandible. The unique feature of the TMJs is the articular disk. The disk is composed of fibrocartilagenous tissue (like the firm but flexible cartilage of the ear) which is positioned between the two bones that form the joint. The TMJs are one of the only synovial joints in the human body with an articular disk, another being the sternoclavicular joint. The disk divides each joint into two. The lower joint compartment formed by the mandible and the articular disk is involved in rotational movement (opening and closing movements). The upper joint compartment formed by the articular disk and the temporal bone is involved in translational movements (sliding the lower jaw forward or side to side). The part of the mandible which mates to the under-surface of the disk is the condyle and the part of the temporal bone which mates to the upper surface of the disk is the glenoid (or mandibular) fossa.