The trunking of electronic telephone exchanges is not a rigid framework which must be specified before the design of an exchange suitable for the rigours of the public service can be undertaken.Changes are repeatedly needed to accommodate other developments, so that a common basis of various arrangements is sought. For exchanges for which a multiplex system of a large number of channels is suitable the basic arrangement which emerges is that of simple inter-highway switching—the system used for the exchange being installed at Highgate Wood.In many of these systems a problem arises because subscribers sharing the use of a multiplex system cannot share the use of a single channel to communicate with each other. From discussion of varioussolutions it is seen that the method of anti-phase channels used in the Highgate Wood exchange, although simple, is unlikely to be repeated, modern transistors making more economical solutions practicable.The problems of traffic-carrying capacity of these exchanges show a wide gulf between the complexity of accurate theory and the relative simplicity of methods which are useful in practice. These methods have been revised, and are presented in relation to their most useful application—the assessment of the relative merits of different trunking systems.