The Lid Master Calipers from Giffin Tec does the work of 2 standard calipers with far greater speed and accuracy. Make one measurement and you have its opposite immediately. The standard Lid Master has a capacity of 11 (28cm). The Large Lid Master, with a capacity of 20 (50.8cm) is available for larger work. The Lid Master works in 2 ways. To make a lid for an unfired pot, measure the opening and tighten the wing nut. Now use the other end of the Lid Master for measuring the lid. Both the inside and the outside ends of the Lid Master will always be identical. b. Now you can throw a new lid for a fired pot without simply guessing the size. Reassemble the wing nut and screw in the 12-1/% hole. Measure the fired pot with the inside end of the Lid Master. Tighten the wing nut. Throw a new lid using the outside end for your measurement. Because good plastic clay shrinks about 12 ½% from we to fired state, the lid will shrink to fit.