This handy, compact caliper is used to measure the outside diameter of any round object up to 13" O.D., including pipes, tubes, discs, flanges and other round objects. It consists of two blades (1"x7.5") joined at one end by a pivot and clamping thumbnut. To use, clamp the blades so that the zero marks meet at the vertex of the angle, place the tool against a round object and read the outside diameter and NPT pipe size where the scales touch the object. On one side the scales are graduated 0 to 6" O.D. and 0 to 5" NPT. On the flip side of the caliper the angle is readjusted to 90 degrees, and the scales are extended to 0 to 13" O.D. and 0 to 12" NPT.