A cuboid block with a very high length accuracy is called a parallel gauge block. They are used for testing and calibrating measuring instruments, as well as for direct measurement. By sliding the blocks together, also called wringing, different length can be made. The adhesion force holds the gauge blocks together. Steel gauges should not be joined together for more than eight hours as they tend to cold weld.
The total deviation is composed of the dimensional deviation of each individual gauge block additively, therefore, combinations should
consist of the least possible amount of individual gauge blocks.
Gauge blocks are linear gauges with the smallest measurement uncertainties. Calibration at a National Metrological Institute
(NMI), in Germany this is the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, uncertainties of a measurement of approximately 20 to 50 nm are achieved.