Arch (the 'arc' line created by teeth in normal position development) is a collective term that describes a variety of appliances used to gain both arch width and arch length. These appliances range from simple appliances with lap springs to high-speed, rapid palatal expanders. They may utilize orthodontic movement and/or orthopedic movement and may be either fixed or removable.

When a patient's arch is v-shaped and needs widening only in the front of the mouth, an appliance that expands only in the anterior (front) region is necessary. An expansion screw in conjunction with a separate posterior (back) hinge in this appliance allows for the gain in anterior space without affecting the posterior bite relationship.