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The upper teeth should fit outside the lower teeth like a lid on a box. If the upper jaw is too narrow, the lower jaw usually swings to one side to allow the back teeth to mesh. A posterior cross-bite results. Posterior cross-bites can result in uneven wear of the teeth and have the potential to promote asymmetric jaw growth in children and jaw joint problems long term. Narrow upper jaws are routinely and easily treated in growing children and this will often be done when patients still have baby teeth in place. Orthodontic treatment involves the use of a fixed or removable appliance to widen the upper jaw (maxillary expansion). Once the upper jaw has been widened, the lower jaw can close normally.