An expander, also called a rapid palatal expander or RPE, is an appliance designed to correct a narrow upper arch that can result in crossbite, crowding, or impacted teeth. Two bands are cemented onto the last upper molars and connected by a metal screw that gets turned regularly to expand the appliance.
While not used as often, a lower expander can move lower teeth that have become tipped in into an upright position. This does not actually widen the bone, but instead just uprights the teeth to improve their position and help alleviate crowding. Additionally, lower appliances commonly accompany an upper RPE.