Wires enter and leave the telephone exchange building in the form of cables (Fig. 95). A cable is composed of pairs of twisted copper wires, insulated with spiral wrapping and enclosed in a lead casing.

Within the exchange these cables are supported by two frames; the main distributing frame and the intermediate distributing frame (Fig. 96). The main distributing frame allows the entering wires of the subscribers' lines to be changed

Fig. 95. - Terminal Room in an Exchange. (a) Relay rack. (b) Intermediate frame. (c) Cables to switchboard without changing the telephone number. The intermediate distributing frame is so constructed as to permit any call to be answered at any portion of the switchboard. Thus no individual operator need be overloaded with calls.