Rapid Palatal Expansion is used to expand the width of the upper jaw. If the upper jaw is narrow it means that the upper and lower jaw do not “fit together” properly, affecting the bite of the teeth. This often results in what is called a crossbite, which can also mean crowding and misaligned teeth. An expansion appliance widens the upper jaw to provide a better fit between the upper and lower teeth, and creates additional space for crowded teeth