1. Meter.
2. Although the SI definition of the «meter» is based on a laboratory procedure combining the speed of light and the duration of a second, a machine shop will have a physical working standard (gauge blocks for example) that is used for checking its measuring instruments.
3. Working standards and certified reference materials used in commerce and industry have a traceable relationship to the secondary and primary standards.
4. Working reference standards.
5. Secondary reference standards.
5.1. Secondary reference standards are very close approximations of primary reference standards.
6. It is a object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity.
7. Primery referends standards.
7.1. Gauge blocks.
7.1.1. They are used as a working standard to check the calibration of measurement tools such as micrometers.
7.2. International prototype kilogram (IPK).
7.2.1. It is a one kilogram mass of a platinum-iridium alloy maintained by the International Bureau of Weights and Measures in France.
7.3. Volt.
7.3.1. Formerly it was defined in terms of standard cell electrochemical batteries, which limited the stability and precision of the definition.
7.4. Meter.
7.4.1. In 1983, the standard mete was redefined as the distance which light travels in a vacuum during 1/299,792,458 of a second.
8. There is a three-level hierarchy of physical measurement standards.
8.1. Primary standards.
8.1.1. They are made to the highest metrological quality and are the realization of their unit of measure.
8.2. Secondary standards.
8.2.1. They are calibrated with reference to a primary standard.
8.3. Working standards.
8.3.1. They are periodically calibrated against a secondary standard.
8.3.2. They are used for the calibration of commercial and industrial measurement equipment.