A grading system that assesses the peripheral iris insertion angle width, at the trabecular meshwork. It relates the extent to which the angle is open in preference to the degree to which it is closed. Shaffer scale grade four (IV) represents a wide-open angle, while grade zero (0) indicates a closed-angle. Angles between 0° and 20° are regarded as capable of closure, while angles between 20° and 45° are considered to have no risk of closure.