The Mitutoyo 619005 gage block holder is used to couple rectangular gage blocks together to increase the length. Holders can also be used with reference points, scribers, and jaw to use as a Go/NoGo (G/NG) gage. The gage block holder couples blocks to an overall length of 153 to 256mm.

Gage blocks (also called gauge blocks, Jo blocks, Hoke blocks, and Johansson gauges) are precision ground and lapped metal or ceramic blocks, used for calibrating precision measuring equipment and as a length standard in manufacturing. Gage blocks are square or rectangular in shape, with flat and parallel measuring surfaces, and usually have an engraved unique serial number and nominal length (provided in either inches or millimeters). They often come in sets with blocks of varying lengths, so that many different standard lengths can be created by stacking and wringing them. Wringing is the process of sliding blocks together with a small amount of pressure, in order to create a longer gage block stack.

Mitutoyo manufactures precision measuring tools, metrology equipment, and related systems. The company, founded in Tokyo in 1934, formed Mitutoyo America Corporation in 1963 with headquarters in Aurora, IL.