The use of roentgenograms for the detection of pes planus constitutes a simple, rapid, and precise method for the determination of the flatness of the longitudinal arch and the various degrees thereof. Roentgenograms are taken with the foot in the lateral position, its external aspect against the film, and the x-ray beam directed from the inner to the outer aspect. A tracing is made from the processed roentgenogram. It is not necessary, however, to trace the whole foot but rather to determine three points. To obtain these, a base line is drawn connecting the plantar surface of the head of the first metatarsal with that of the os calcis (Diagram 1). A is the point of contact of the head of the first metatarsal with the base line, while C is the point of contact of the os calcis with the same line. B is the plantar surface of the base of the first metatarsal bone. In patients whose feet are not absolutely flat, these three points will not be on a straight line. Euclidian plane geometry has taught us that th...