The application of certain basic principles of switching and logic system design in electronic telephone switching systems is illustrated in an outline description of a fully-solid-state switching system for exchanges having some 10 000 lines and 1000 junctions. Speech transmissionis ensured on a one-wire time-multiplex p.a.m. basis using the principle of resonant transfer between modem circuits.A particular arrangement of the highway interconnecting network reduces the total number of cross-points to less than 1.2 per line and 2.5 per junction for speech and signalling. Although unbalanced multiplex highways are used throughout, the problem of inter-highway crosstalk can be mastered without particular difficulties, while the overall transmission losses can be reduced to 1.7 dB.Central control has been conceived as a separate data-processing system using ferrite stores and semiconductor logic at relatively low speeds, determined mainly by the real-time requirements of dial-pulse detection and supervision.Outstanding flexibility and design facility has been achieved by the large-scale use of semi-permanent information stores for translation, hunting and programme-controlled logic.