Gage blocks are the most used physical representation of length available to companies requiring the highest order of precision for their work. Their versatility makes them indispensable when ‘master’ dimensions are required for checking work or for setting or calibrating instruments. To the uninitiated, they look like rectangular or square blocks of metal with two polished faces, but their simple appearance does not reveal the precision to which the length between those two polished faces has been refined.
The unique properties of gage blocks means little unless they are well maintained and calibrated at intervals appropriate to their use. Calibration of them is an exacting process with the highest level of work being performed by NIST and a few industrial facilities. But the cost of achieving this level is beyond the reach of most budgets so companies invariably turn to commercial laboratories for the service where the uncertainties involved are higher but the cost is considerably lower.