Although arch development is more commonly used in the treatment of developing children, arch expansion appliances can be successfully used to treat adults. They are particularly effective when the posterior teeth are lingually tipped in conjunction with the arch being narrow. A typical custom appliance, consisting of lingually tipped posteriors and crowded anteriors, is shown here. When using this appliance with an adult dentition, lateral development can be greatly enhanced by the addition of a second expansion screw. This ensures uniform anterior-posterior pressure and affords more stability as the screws reach the limits of their threads. Note: A small amount of differential expansion can be accomplished by adjusting these screws at a different rate. Care must be taken, however, to avoid cracking the appliance.