Measurement is the fundamental activity of inspection. The intent of inspection is to ensure that what is being manufactured will confirm to the specifications of the product. The examination of the product either during or after manufacture, either manually or automatically, falls in the province of inspection. Basically, inspection of products can be done in two ways:

1.By attributes, with the use of gauges to determine if the product is good or bad resulting in a yes or no, go or no-go decision.

2.By variables, with the use of calibrated instruments to determine the actual dimensions of the product for comparison with the desired size.

The four fundamental measures on which all others depend are length, time, mass, and temperature. These four measures, along with the ampere and the candela, provide the basis for all other units of measurement. Most mechanical measurements involve combinations of units of mass, length, and time.

Precision measurement is the key to producing interchangeable parts and mass producing goods. Every part must be made accurately, within specified limits, to the size and shape specified by the designer. Inaccurately made parts will not assemble and fit properly with mating parts. Hence, the finished product may not operate properly, or may wear out sooner than it should.

All workers in the machine shop must be responsible for accurate work. Accurate workmanship depends primarily on accurate measurement and layout work. To ensure accuracy, machinists must know the principles of measurement. They also must know how to use the common hand tools, measuring instruments, and gauges used in the trade.

Machinists use many instruments and gauges for making measurements. Some of these measurements need only be accurate to half a millimeter. At other times, measurements must be made within two-hundredths of a millimeter or less. In the manufacture of certain gauge blocks and measuring instruments, measurement must be made to within 50 μmm (2-millionth of an inch). To make accurate measurements, the machinist must learn how to use tools such as rules, micrometers, vernier calipers, gauge blocks, and special optical instruments.