Incongruency angle is determined by drawing line joining lateral extent of talar articular surface (point A) and lateral extent of navicular surface (point B). Second line is drawn between lateral aspect of talar neck at its narrowest segment (point C) and lateral extent of talar articular surface (point A). Distal and lateral interval between these two lines forms incongruency angle. A, Incongruency angle in normal foot. B, Incongruency angle in type IIb flatfoot deformity.