An appliance used to move teeth as part of orthodontic therapy. Movement of the teeth may be achieved by intra-oral or extra-oral traction. Fixed appliances are attached directly to the teeth. The components of fixed appliances, namely bands or brackets, adhere to the tooth surface and archwires attached to them apply a force to the teeth and are capable of changing the mesio-distal angle of the teeth (tipping), changing the bucco-lingual inclination of the teeth (torquing), rotating or bodily moving teeth. They utilize archwires and auxiliaries (e.g. ligatures, elastics, springs, separators). Fixed appliances have an advantage over removable appliances in that they can be used for multiple tooth movement and they make it possible to exercise precise control over force distribution to individual teeth. Examples are the edgewise, Begg, and tip-edge appliances. Removable appliances are capable of being removed by the patient for cleaning. They can be used as active appliances by utilizing springs, wires, bows, screws, elastics, or the acrylic resin baseplate. A functional appliance ( myofunctional appliance) is used to correct jaw disharmonies by modifying the growth of the jaws in an actively growing patient by utilizing the forces generated within the masticatory and facial muscles. They act on both upper and lower teeth at the same time and may be either fixed or removable. Changes induced by functional appliances are thought to be due to changes in the dento-alveolar complex, skeletal changes, or changes in the glenoid fossa of the temporomandibular joint. There are many different appliance types used to treat specific malocclusions; these include the Andresen, Bimler, Frankel, monobloc, and twin block appliances. A lingual orthodontic appliance is a fixed orthodontic appliance placed on the lingual or palatal tooth surface. A vacuum-formed appliance is usually made of clear plastic, using a ‘suck-down’ machine that forces the plastic to adapt to the contours of the model; it is often used as a retainer following active treatment but may also be used for minor tooth movements.