Threaded plug gages are calibrated at Griffin Incorporated using the "three wire" method. In this scenario, a direct measuring device like a SuperMicrometer or a LabMaster is mastered within the range of the UUT (unit under test). The thread plug gage being measured/calibrated (the UUT) has a designation such as 1/4-20 unc-3B meaning that it has a basic diameter of 1/4", it has a pitch of 20 threads per inch, it is a unified coarse pitch and it is a class 3 gage for the inspection of female threads.
We would select the appropriate thread wire set, one designed for the inspection of 20 pitch threads, and place 2 wires on adjacent threads and the 3rd wire directly opposite the first two. The UUT and the wires would then be measured and the constant for the wires would be subtracted from the measurement over wires, yielding the UUT's pitch diameter. The pitch diameter is the effective diameter of a threaded artifact.