A retainer refers to an orthodontic appliance that is designed to help the teeth “retain” their current position in the mouth. As the teeth settle into the soft tissue and jawbone that keep them in place, they are prone to shifting over time. Retainers prevent the movement of the teeth as a patient ages. A retainer can be made of wire and plastic or just solid plastic. A retainer is custom-made to perfectly fit the mouth and teeth of the patient. A retainer made from solid plastic is usually completely clear, but it will also likely be more expensive. A retainer can be worn over the teeth or behind the teeth, depending on the type.