The Twin Block Appliance differs from other functional appliances in that it is actually two separate appliances (maxillary and mandibular) working together to maintain an advanced mandibular position. This is achieved by incorporating acrylic blocks into the occlusal surface of the appliances which interface with each other in the bicuspid region at a 70 degree angle. This interface prevents the mandible from sliding back into its accustomed position. The reduced bulk of the appliance allows patients to wear the Twin Block full-time including while eating thereby harnessing the forces of mastication and occlusion for faster treatment results. As treatment progresses, the orthodontist can selectively reduce the bite surfaces to allow for the desirable eruption of posterior teeth.