The jaw joint (called the temporomandibular joint) is the only joint in the body that rotates and displaces its position at the same time. The following picture shows the change in internal structure of this joint when a person opens his/her mouth.

This is made possible by a meniscus (referred to as a disc in the first picture). This ‘disc’ is held in place by ligaments. Have you ever wondered why your jaw clicks when you open your mouth? Grinding or clenching your teeth while sleeping will cause stretching of the ligament that holds the ‘disc’ in place.