Generally, every gage will need a master to set/calibrate the gaging member and the air gage readout. This ensures dimensional data is both precise and quanitative. Masters are generally plugs, discs, rings, or set probes. Master plugs and discs are used to calibrate air rings, snap gages, and other external measurement instruments. Master rings and set probes are used to calibrate spindles, air probes, and other internal measurement instruments. By simplest definition, a master is used to dictate how much back-pressure indicates the internal or external measurement is closest to “the perfect dimension”.