The twin block was developed by William Clark for the treatment of Class II malocclusions. It was based on the same theory as the monobloc and Frankel appliance. The appliance consists of maxillary and mandibular bite blocks designed to use the forces of occlusion as the functional mechanism. As the appliance was formed of two separate pieces, it could be worn full time and was more comfortable for the patient. Since then, the twin block has undergone many adaptations. Clark has since developed TransForce Lingual Appliances. A lingual appliance pre-activated with nickel titanium springs for transverse and sagittal arch development.