A gauge block is a block of metal or ceramic with two opposing faces ground precisely flat and parallel, a precise distance apart. Standard grade blocks are made of a hardened steel alloy, while calibration grade blocks are often made of tungsten carbide or chromium carbide because it is harder and wears less.
Gauge blocks come in sets of blocks of various lengths, along with two wear blocks, to allow a wide variety of standard lengths to be made up by stacking them. The length of each block is actually slightly shorter than the nominal length stamped on it, because the stamped length includes the length of one wring film, a film of lubricant which separates adjacent block faces in normal use. The thickness of the wring film is about 25 nanometers (0.98 μin). The gauge’s nominal length is also known as the interferometric length.