Tongue thrust is involved in nearly all open bites. An open bite can be created by tongue thrust, tongue posture or mandibular posture. Tongue is an unusual muscle in that its contraction allows it to assume many shapes; its influence in the swallow can create an open bite in the area dictated by the contraction, thrust or rest position. Once space has been created between the upper and lower teeth by the tongue it continues to enter the space created, consequently enlarging the space. Skeletal open bites do not occur in patients whose masseter muscles are active during the swallow (an open bite can develop in these individuals due to habit or during orthodontic therapy). In these individuals the masseters fully contract during the swallow. The weaker the masseter muscles, the more likely an open bite will either be present or may develop during orthodontic treatment. Orthodontic movement will always result in occlusal interferences at some point in treatment. Occlusal interferences during orthodontic treatment make it difficult for patients to find a comfortable biting position. At this point they do not fully contract--"squeeze"--the masseters during the swallow to avoid traumatizing the teeth. Once the masseter squeeze is reduced, the tongue must contribute more to the swallow than when the masseters were more actively involved. The tongue becomes the cushion for the dentition during the swallow. The tongue now positions itself between the teeth during the swallow and the open bite during orthodontic treatment is born. Once the open bite occurs it is best to treat it immediately as the open bite thus created will only worsen with time. If the patient was originally a counterclockwise grower (brachycephalic), it is easier to correct. A clockwise grower (dolichocephalic) is hardest to correct, but can be corrected with patience, perseverance, exercise and a properly constructed tongue thrust appliance. While it is true that some skeletal configurations (i.e. clockwise growers, dolichocephalic facial types) lend themselves to the development of an open bite, the skeletal configuration itself seldom produces an open bite. The tongue is the main progenitor of open bites and it is also responsible for perpetuating the open bite. Dolichocephalic facial types lend themselves to open bite development as any orthodontic treatment which either distalizes molars or allows molars to extrude will tend to wedge the mandible and dentition open. (In this type of patient the masseter muscles develop insufficient force on closure to intrude the molars.) Once the bite opens, the tongue now enters the picture in order to complete the swallow. The open bite immediately worsens. The open bite thus created may require several months to correct. In the clockwise growth patient it is imperative that all precautions be taken during treatment to counteract open bite development. It is also imperative that an open bite, once detected, be treated immediately as the longer it persists the more difficult it becomes to treat. If left untreated, it becomes a habit and the more engrained a habit the more effort needed to change it.