The Twin Block is a removable functional appliance consisting of two bite blocks (twin blocks), upper and lower, that are designed to interlock at 70 degrees in such a manner that the mandible is held in a more protrusive position. Over time, the repositioning of the mandible forward eliminates the overjet and, when acrylic is removed from the upper block, eruption of the lower first molars occurs eliminating the overbite. When treatment with the Twin Block is complete, the first molars will be in contact and the maxillary and mandibular incisors will be nicely coupled. In the mixed dentition sample shown here, the lower molars will usually erupt passively. But in the permanent dentition, the lower molars often need to be actively erupted using vertical elastics.