Anterior cross bites occur when the upper front teeth are inside of the lower front teeth. Over time, the lower jaw grows into a forward position abnormally, thus inhibiting the growth of the maxilla (roof of the mouth or upper jaw). Once the source of the interference causing the cross bite is eliminated, the appliance guides the maxilla back into position. The front of the appliance is moved outward through the tension of an expansion screw at the base of the unit while the back teeth are used for anchorage.