incisors is counter-acted by an opposite and equal extrusive force acting at the posterior teeth of the same arch. The extrusive force is felt at buccal of the center of resistance of a molar tooth because the bracket slot in the molar tube or band lies buccal to the tooth. Thus, there is a moment that is produced that leads to lingual crown tip and buccal root tip of that molar tooth. This effect can be dealt by using a Lower Lingual Holding arch or a Transpalatal Arch to control the transverse side-effects. One-couple vs. Two-couple System In a one-couple system there is a single point of application of force system which allows one to devise what type of force is being generated. For example, in a segmental intrusion arch, the arch is tied to a base archwire anteriorly and engaged into a tube in the posterior molar. This produces only a force in the anterior but a force and a couple in the posterior at the molar. Thus a couple is generated only at one end of the intrusion arch and thus this type of system is known to be one-couple system which can be statistically determinate. If the intrusion arch in engaged in the slots of the anterior brackets and at the tube in the posterior teeth, then a two-point contact system is generated which prevents the clinician from determining what type of forces are being generated clinically. This type of system is known as statistically indeterminate system and can be seen with intrusion arches such as Rickett's Utility intrusion arch. See also Malocclusion Open Bite Malocclusion Charles J. Burstone References Orthodontics