A proctator is a device made of paper, plastic, or metal, cut in a full circle or semicircle, with angle graduations along the circumference. A fine point identifies the circle center. The protactor is centred at the angle vertez with the zero line along one side and the proper angle point marked at its edge. Protectors are available in sizes having radii from 2 to 8 in. or more. A metal circle with a movable arm extending beyond the edge is convenient. The arm rotates about the center and has a vernier reading to minutes. Protactors do not generallyyield accuracies equal to thse of tangent or chord methods.