Wave guides, though known in principle for many years, became of practical importance only with the advent of centimetre wavelengths, and are now, because of their low attenuation and high power-handling capacity, the standard feeders for these wavelengths. They only transmit energy freely above a critical frequency. Energy may then be propagated in one or more modes, and single-mode propagation is desirable for practical purposes; this is achieved by the use of wave guides of rectangular cross-section. Transmission-line theory can usefully be applied to wave guides.The use of wave guides involves a number of auxiliary components such as couplings, bends, corners, branches, switches, rotating joints, etc., whose design involves specialized techniques. The use of a single aerial and wave guide for both transmission and reception has led to close integration of wave-guide technique with transmitter and receiver design; there is similarly a close integration of wave guide and aerial techniques.