The Twinblock was first introduced by W. Clark and is the last and, probably, most powerful development of numerous double plates. These are, in principle composed of a maxillary and mandibular part – unlike the Activators which are a monoblock construction. All these appliances are designed to bring the lower jaw forward in cases of distal occlusion (Angle Class II). The double plates are suitably connected by a mechanical element. These are usually stable metal spurs as with older appliances with Müller-thorns or younger advancement double plate. Advantage of the double plates is usually somewhat lower speech impairment by their reduced volume and the fact that they can keep the lower jaw in advanced position even in case of slight mouth opening. As to monoblocks (Activator, Bionator etc.) the lower jaw sags behind when the mouth is slightly opened which can make these appliances ineffective. The Twinblock consists of two slim plastic plates which are located exactly between the rows of teeth and form an inclined plane.
The Twinblock is basically used for all-day wear – except for meals and oral hygiene – and is often used for some weeks cemented onto the teeth. If a child is actually wearing this appliance, the Twinblock is nearly as effective as the Herbst Appliance. Unfortunately 30-50 % of children do not wear these appliances as instructed so that the Twinblock does not come up to the effect of fixed appliances as the Herbst-Appliance in real life. Nevertheless, it is probably one of the few removable appliances which can be used, even today, with clear conscience.