Humans have one mobile jawbone known as the mandible or sometimes commonly referred to as the lower jaw. The corresponding upper bone is part of the skull and know as the maxilla, although it sometimes referred to as the upper jaw. In order for the mouth to open or to bite down or chew, the lower mandible articulates with the temporal bones of the skull at the temporomandibular joint (TMJ). This is a gliding joint on either side of the lower part of skull, just slightly in front of the ear. It is only the lower mandible that moves and not the upper maxilla as sometimes thought.
In childhood while the deciduous (milk) teeth are still present, there are 20 teeth altogether – 10 in the maxilla and 10 in the mandible. Gradually this is replaced by the permanent teeth with a full complement consisting of 32 teeth – 16 in the maxilla and 16 in the mandible. The jaw is closed tightest when biting and chewing by forceful contraction of the muscles of mastication. The teeth of the maxilla and mandible make contact in a an alignment that ensures the upper teeth fit slightly over the lower teeth. This normal alignment is known as occlusion. The contact between the upper and lower teeth when chewing occurs only for short periods of time. The teeth being hard structures do not experience much wear and tear with chewing apart from when biting and grinding hard foods. However, if the teeth are constantly in contact and grinding against each other, it can become eroded over time.