R ESEARCH into the origin of abnormal narrowness of the jaws, especially the maxilla, which we carried out by means of many individual and serial examinations of children during infancy and early childhood, has shown that every marked alteration of the jaw due to inhibition of growth or deformation is not a local symptom but merely a characteristic of a complex anomaly which usually extends beyond the immediate region of the jaws. The proximity of t.he, maxilla and the palate to the region of the nose and the sinuses makes it clear that every deformation of the maxilla coupled with a pathologic high palate must be accompanied by a retardation of growth and narrowness of the nasal cavity. The strong developmental stimuli which the masticatory organ imparts to the middle face also explain that in most cases of severe underdevelopment of the jaw and correspondingly reduced function, the facial skull also shows definite signs of underdevelopment. These relationships are well known to every orthodontist, and I am only pointing them out because they are too often forgotten in the effort to create a dent.al arch which is as harmonious as possible. On the other hand, during the course of daily practice one is constantly reminded of these important relationships. The ear, nose, and throat specialist often enough refers his mouth-breathing patients for orthodontic treatment when he has realized that the malocclusion and, above all, the narrowness of the jaw are the chief cause for the impeded nasal respiration and that his own treatment is no longer sufficient to prevent the recurring diseases of the respiratory tract. In many cases free nasal, respiration can be achieved only by an extensive widening of the maxilla and the palate. The patient often registers the widening of the nasal passage with the happy statement: “At last I can really get air!” In many, but not all, cases the widening of the jaw has to be coupled with a movement of the mandible into the correct bite.