Each line circuit is marked with signals directly indicative of its number. These are applied through two non-linear resistors which function as static switches and offer a much higher resistance to the direct biasing voltage than to the superimposed 750-c/s signals.The two resistors, which are connected to each line, are interconnected in the form of a co-ordinate system, which provides an almost static marking distribution.The signals pass along the connection to the point requiring the identification, where they are decoded by a standard 2-v.f. receiver and an all-relay distributor. The equipment functions satisfactorily at 50 impulses/sec, making it possible to identify any subscriber's line on a 10 000-line exchange in less than half a second.