In radio trunk systems a number of two-way radio channels are used over a common route with repeaters provided at intervals. Each radio channel carries a baseband signal by frequency modulation of the radio carrier frequency. The baseband signal may consist of an assembly of single-sideband telephony channels and/or a television channel.When choosing the frequency pattern for the radio channels, selectivity requirements are found to arise, not only from the primary fact that the channels are adjacent in the frequency spectrum, but also from secondary features of the equipment such as, for instance, the intermediate frequency used in the heterodyne repeaters.The paper considers the effects which give rise to such selectivity requirements and shows how to choose the channel frequency pattern and the intermediate frequency to minimize the selectivity requirements. The choice takes account of amplitude non-linearity in waveguide circuits. The presence of this was expected on theoretical grounds and has been confirmed by measurements.A specific frequency pattern is worked out for a route with six two-way channels in which each radio channel can carry at least 600 telephone channels or a single television channel. This arrangement was adopted by the C.C.I.R. in 1956.