teeth present in the mouth, the natural vertical dimension of occlusion is lost and the mouth has a tendency to overclose. This causes the cheeks to exhibit a "sunken-in" appearance and wrinkle lines to form at the commissures. Additionally, the anterior teeth, when present, serve to properly support the lips and provide for certain aesthetic features, such as an acute nasiolabial angle. Loss of muscle tone and skin elasticity due to old age, when most individuals begin to experience edentulism, tend to further exacerbate this condition. The tongue, which consists of a very dynamic group of muscles, tends to fill the space it is allowed, and in the absence of teeth, will broaden out. This makes it initially difficult to fabricate both complete dentures and removable partial dentures for patients exhibiting complete and partial edentulism, respectively; however, once the space is "taken back" by the prosthetic teeth, the tongue will return to a narrower body. Vertical dimension of occlusion As stated, the position of maximal closure in the presence of teeth is referred to as maximum intercuspidation, and the vertical jaw relationship in this position is referred to as the vertical dimension of occlusion. With the loss of teeth, there is a decrease in this vertical dimension, as the mouth is allowed to overclose when there are no teeth present to block the further upward movement of the mandible towards the maxilla. This may contribute, as explained above, to a sunken-in appearance of the cheeks, because there is now "too much" cheek than is needed to extend from the maxilla to the mandible when in an over closed position. If this situation is left untreated for many years, the muscles and tendons of the mandible and the TMJ may manifest with altered tone and elasticity. Pronunciation The teeth play a major role in speech. Some letter sounds require the lips and/or tongue to make contact with teeth for proper pronunciation of the sound, and lack of teeth will obviously affect the way in which an edentulous individual can pronounce these sounds. For example, the consonant sounds of the English language s, z, j, and x are achieved with tooth-to-tooth contact; d, n, l, t, and th are achieved with tongue-to-tooth contact; the fricatives f and v are achieved through lip-to-tooth contact. The edentulous individual finds these sounds very difficult to enunciate properly. Preservation of alveolar ridge height The alveolar ridges