When machine parameters are expressed in a properly selected per-unit system, the parameters of a wide range of machine sizes fall into relatively narrow ranges even though the actual values of these same parameters may vary over extremely wide ranges. The per-unit system thereby makes possible an easy comparison between different machines, essentially independent of size, without the use of conversion factors, and facilitates the identification of gross calculation errors. The proper selection of base values is fundamental to the usefulness of a per-unit system. Although per-unit quantities have been used for many years by designers of rotating machinery, the base quantities were not standardized, and it was necessary to define them in each publication. A working group of the Rotating Machinery Committee was created to propose a set of standard definitions. In 1961, a proposed standard (AIEE No. 86) was published for trial use. A critical review of that proposed standard has resulted in this present standard.