Dance master (measuring tool)
Tanzmeister or combination internal external buttons are metalworking test devices made of steel . They serve as measuring tools for hard-to-reach outside diameters and wall thicknesses as well as for measuring inside diameters .
Dance masters were used for testing cannon barrels and cannon balls . With the dance master, the balls could be compared directly with the inside diameter of the pipe so that no pipe cracks could arise from a cannonball that was too thick .