Fig. 34. Geometry of pulley function and implementation of Listing's law by the half-angle rule. A. The eye in primary position. The pulley is located as far posterior to the axis of rotation as the muscle insertion is anterior to it. B. Elevated gaze. The axis of horizontal rotation tilts back at half the angle of gaze elevation. C. Adduction of the eye. The pulleys move as far posterior to the axis of rotation as the muscle insertions move anterior to it, preserving half-angle behavior.