It is generally thought that during symmetrical jaw opening the mandible rotates about an axis passing through the centres of the two mandibular condyles, and translates down the articular eminence. The author argues that there are no data to support this assumption. The true combination of movements can only be understood by understanding the constraints that operate during jaw opening and these are the articular eminence and the temporomandibular (TM) ligament. The TM ligament is pulled taut during jaw opening, and so acts like the chains of a swing: the mandible rotates about the lowest attachment of the TM ligament to the condyle and swings about the most posterior attachment to the articular tubercle. The effects of combining the swing and rotation are demonstrated by a simple mechanical model. The functional advantage of the TM ligament is that it keeps the condyle close to the temporal bone during jaw opening, until the condyle passes under the articular eminence. The constraints described in this paper may have important implications related to the diagnosis and treatment of some disorders of the TM joint (to be described in a later paper).