T HE treatment of malocclusion consists of changing the abnormal relationships of the teeth and the jaws to a normal relationship. A change in relationship implies a change in position, and a change in position represents motion. Motion can be imparted to a body at rest by the application of a force; accordingly, the teeth which are at rest in relation to the supporting structures can be made to move by exerting controlled pressures upon them. These controlled pressures, or applied forces, may be of several different characters. The various mechanical devices in use at the present time apply forces to the teeth in different manners, and thus bring about different kinds of tooth movements. The recognition of the various kinds of tooth movements induced by these mechanical devices is very important, for the intelligent selection of a working appliance depends upon a full understanding of appliance action. Thus, it is made clear that the selection of an appliance is governed entirely by the required tooth movements. It has never been pointed out that there is an inherent tendency on the part of the teeth to move into normal positions. While this does not always take place, this tendency is clearly recognizable in all instances. Malocclusions are the result of the failure of the teeth to move into normal positions. The development of 6he specialty of orthodontics may be traced directly to this characteristic tendency of the teeth to align themselves properly. The early orthodontic appliances depended almost entirely upon this tendency. The plain labial appliance is a final expression of these earlier attempts, and its successful operation depends upon the ability of the teeth to move into normal positions. It is possible to bring about all the required tooth movements by the plain labial appliance, such as buccal or labial, lingual and mesiodistal tipping movements, bodily movements, root movements and rotations; but many of these changes cannot be explained on a mechanical basis and cannot be directly attrihut,ed to the appliance. As a result of this characteristic property of the teeth, a very large number of cases can be successfully treated by the plain labial arches, and with little effort almost all movements may be effectively produced. There are a number of instances, however, where the plain labial appliance proves to be inadequate. The inefficiency of the appliance and the lack of force control become apparent, and other devices must be resorted to in order to attain a desirable result.