Fresh anatomic specimen forearms were studied using a mechanical device, the axis finder, to locate the axis of rotation. The relationship of the axis to the membrane was demonstrated directly using a small jig. The axis of rotation of the forearm is constant and independent of elbow flexion or extension. It runs from the center of the radial head to the center of the distal ulna. All fibers of the interosseous membrane cross the axis of rotation near their distal insertion into bone. This relationship of the ligaments to the axis of rotation is similar to those of the ankle, knee, and thumb joints. The membrane does not limit forearm rotation and can provide little stability if the bony ring is disrupted.