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At the start of the 20th century, the five different frequencies of 25, 30, 40, 50, and 60 cycles were in use, to a greater or lesser degree, in the United States. There was also a smattering of other oddball frequencies. As a result of the number of frequencies in use, the concept of the rotating "frequency changer" was developed early in the last century in order to be able to interconnect systems operating at two different frequencies. This article describes the development of the rotating frequency changer and its application, over the past century, in a variety of utility, industrial, and transportation power systems. In particular the induction-synchronous frequency changer is discussed.