The transmission of speech over long trunk routes is operated on a multi–channel carrier basis employing frequency division multiplex. The usable frequency spectrum of the line or cable is split into a number of adjacent bands, each of which is then employed as a separate channel. Modern systems are usually operated on a four-wire basis, which comprises a transmit channel and a receive channel, the two channels then form one circuit. Of all the systems in current use, the coaxial cable system is most important, as it forms the basic system for national network in all industrial countries. Twelve and twenty four channel systems are so arranged that they may form part of the coaxial cable system.