Set of Johanson gauge blocks in an oak case, c 1900.

Gauge blocks were developed in the 19th century to provide an accurate and consistent means of measuring length and are still used as the industry standard today. A combination of blocks is stacked until the required length is reached. The blocks are then rubbed against each other to squeeze out any air. The thin film of preservative oil or moisture on the blocks and the molecular bonding between the surfaces of the blocks holds them together. The length of the stack is then compared to the target length. Gauge blocks can be made from a range of different materials which offer varying levels of accuracy, depending on factory requirements.