A functional appliance is a type of brace to correct a “Class II” problem; Class II problems are a group of bite problems where the top teeth bite in front of the lower teeth. Functional appliances may be either fixed or removable.

Removable functional appliances are most popular in the U.K. The more common removable functional appliances are made from separate upper and lower parts, which interlock placing the lower jaw in a more advanced position. Other types of functional appliance are made from just one piece, although these may adapt slightly less well to the teeth.