The periodontium is a membrane-like structure between the tooth and the alveolar bone. Its main function is to act as a cushion for the tooth. Because the tooth itself is a bony structure and the bone is also bony, two hard structures that grow directly together can cause trauma to the bone when biting, and in this case the alveolar bone will shrink. The role of the periodontium is to act as a shock absorber when biting, so that the biting force is not transmitted directly to the alveolar bone, preventing inflammation. The periodontium is not normally visible or felt. Only during a hospital examination, such as an x-ray, can the periodontium be seen as a very thin line of structure. If the periodontal membrane is normal, it will look as thin as a hair on the film. If there is periodontitis, or if this tooth is traumatized, the periodontium will widen and may be about 1mm wide.