The waveforms which represent speech and telegraph signals are of complex shape. Nevertheless if they repeat the same pattern, complex waveforms can be analysed into two or more components each of which is itself of pure sine-waveform. Due to this reason, ac theory and transmission line theory have been developed on the assumption of a pure sine-waveform.
In any complex waveform the lowest frequency component is known as the fundamental frequency ; other sinusoidal components whose frequencies are exact multiples of the fundamental frequency are called harmonics. The first harmonic coincides with the fundamental frequency.