The sinusoidal waveform, at a given frequency, is characterized by its amplitude and phase. In a single-phase source we studied so far, it is assumed that a sinusoidal wave is the input. However, keeping the amplitude the same, a source with three sinusoidal waveforms differing in equal amount of phase turns out to be better for electric-power generation, transmission and distribution systems, and in various other practical applications. The resulting circuits exhibit symmetry both with respect to geometrical structure and element values. Circuits with three sources differing in phase by 120∘ and of the same amplitude, called three-phase circuits, are mostly used in electrical power systems.