Functional appliance refers to a large and diverse family of orthodontic appliances designed mainly to correct skeletal Class II malocclusion.  A functional appliance is a removable or fixed orthodontic device that uses the facial and jaw muscles to improve the way that the teeth bite together. FEATURES Functional appliances all have a postural effect on the mandible and it works on cases where there is reterognathic mandible Appliance work by a combination of altering the soft tissue envelope that surrounds the teeth, disrupting the occlusion and by creating an intermaxillary force. Removable Functional appliances include Activator, Bionator, Frankel Appliance, Twin Block and Fixed include Herbst appliance, Jasper Jumper.