Radiograph showing the relative position of point A. Point A is used to measure the modified Tönnis angle, and the relative position of point A is determined by angle B. R1 and R2 represent the radii of the femoral head and acetabulum, respectively. Theoretically, cos. B = R1/R2; therefore, the relative position of point A can only be affected by the radii of the femoral head and acetabulum.