Consider the second harmonic with frequency 2f. In one cycle of the fundamental vibration (which takes a time T) the second harmonic has exactly enough time for two vibrations. The third harmonic has exactly enough time for three vibrations, and the nth harmonic has exactly enough time for n vibrations. Thus, at the end of the time T, all of these vibrations are 'ready' to start again, exactly in step. It follows that any combination of vibrations which have frequencies made up of the harmonic series (i.e. with f, 2f, 3f, 4f, .... nf) will repeat exactly after a time T = 1/f. The harmonic series is special because any combination of its vibrations produces a periodic or repeated vibration at the fundamental frequency f. This is shown in the example below.