Each connexion involves the use of three switches, viz., a first selector, a connector switch, and a second selector which is brought into operation between the other two.
Each subscriber's circuit on this system terminates upon the incoming portion of a selector switch, called a first selector, and is multipled upon the outgoing portions of a number of similar switches called connector switches.
Only calls originated by a subscriber pass through the selector switch (first selector) provided for his sole use; the calls incoming to him pass through one or other of the various connector switches upon which his circuit is multipled.
By means of his first selector the circuit of a calling subscriber is connected to the outgoing end of a junction whose other end terminates upon the incoming portion of a second selector in the thousand group to which the wanted subscriber belongs.
The second selector in turn extends the connexion by means of another j unction circuit to one of the connector switches in the hundred group wanted, while finally the connector) switch completes the connexion.