The zygapophysial joints are intervertebral articulations between the two superior articular processes of a vertebra and the two inferior articular processes of the overlying vertebra. The cartilaginous surfaces of each joint are enclosed by an articular capsule, which is relaxed in the cervical and tense in the lower vertebral region. There are no accessory ligaments. The joint articular surfaces vary from region to region. In the cervical region they are planar and primarily situated in the transverse plane. In the thorax they are planar and situated in the frontal plane. The surfaces of the lumbar vertebrae are reciprocally curved, one surface being concave and the opposite convex. They are primarily situated in the sagittal plane.