Johansson gauge blocks, also known as jo blocks or gauge blocks, are precision measuring instruments used in metrology to determine length or distance. They are made from a high-carbon steel or ceramic material, and are carefully ground and lapped to a precise size, typically to within millionths of an inch.

The blocks are stacked together to form a measurement of a desired length, and their precise size and flatness allow for accurate and repeatable measurements in many different applications, including inspection, setting up machine tools, and testing and calibrating other measuring instruments.

Johansson gauge blocks come in sets that include blocks of various sizes, ranging from 1 millimeter to 300 millimeters or more, and are used in a variety of industrial and scientific applications, including manufacturing, engineering, and quality control. They are considered a key tool in dimensional metrology, and their accuracy and versatility make them a widely used and essential tool in many precision measurement applications.