This Topic looks at the case of calculating the fundamental frequency or period of a periodic waveform, where this is not known in advance. The Topic also considers, as a consequence, whether the sum of periodic signals results in a periodic signal. The calculation is relatively straightforward, but needs some care. Several detailed examples are provided for finding the fundamental frequency, if it exists. If a finite arbitrary sum of sinusoids and co-sinusoids is provided, then it is demonstrated how the Trigonometric Fourier coefficients can be obtained by inspection.