The object of the paper is to explain in common-sense terms the method of calculation developed in 1918 by Mr. C. Le Geyt Fortescue and generally known as the method of symmetrical components.This method shortens and simplifies the solution of problems of unbalanced polyphase systems. It has made practicable the calculating board by which such problems may be solved automatically.In order to demonstrate its wide usefulness, the method of symmetrical components is here used in an explanation of the action of the phase convertor.