A paper by Lord Rayleigh, published in February 1897, was seminal in establishing the waveguide as a possible transmission medium which was `out of the ordinary' for the time and, in the event, some years ahead of its practical application. The waveguide is now well established as an engineering tool, especially where there is a requirement for high power along with high sensitivity and low loss. The behaviour of waveguides in the below cut-off region, where the attenuation does not become infinite but reaches an asymptotic value dependent on the guide dimensions, is now the basis of a primary standard of attenuation used in standards laboratories throughout the world. Despite the high attenuation, `propagation' in a waveguide in the cut-off mode has attracted interest recently through claims that the group velocity may be of the order of four times the velocity of light.