It is important to recognize the development of an arch-width problem in pediatric patients and to determine the best time to treat it. One of these conditions is the posterior (buccal) crossbite where one may also find an exaggerated overjet, caused by maxillary excess, and/or mandibular width deficiency. One may also find a mandibular midline deviation on the side of the crossbite, creating a long-term orthopedic problem with a mild facial asymmetry. When correction of this condition is attempted in adulthood, poor results can be expected, making this the most important reason for early treatment. Six clinical cases are presented, along with the mechanics performed in both arches