An electrical system or apparatus that has two or more alternating supply voltages displaced in phase relative to each other. In a symmetrical polyphase system each voltage is of the same magnitude and frequency and is displaced by an equal amount. If there are n sinusoidal voltages the mutual phase displacement is 2π/n radians and the system requires n lines at least. Thus in the three-phase system there is a phase difference of 2π/3 between three voltage lines. An exception is the two-phase system that has a phase difference of π/2 between the two voltages.