Trunk Lines.  A trunk line is a line which is not devoted to the service of any particular subscriber, but which may form a connecting link between any one of a group of subscribers’ lines which terminate in one place and any one of a group of subscribers’ lines which terminate in another place.  If the two groups of subscribers’ lines terminate in the same building or in the same switchboard, so that the trunk line forming the connecting link between them is entirely within the central-office building, it is called a local trunk line, or a local trunk.  If, on the other hand, the trunk line is for connecting groups of subscribers’ lines which terminate in different central offices, it is called an inter-office trunk.

Toll Lines.  A toll line is a telephone line for the use of which a special fee or toll is charged; that is, a fee that is not included in the charges made to the subscriber for his regular local exchange service.  Toll lines extend from one exchange district to another, more or less remote, and they are commonly termed local toll and long-distance toll lines according to the degree of remoteness.  A toll line, whether local or long-distance, may be looked upon in the nature of an inter-exchange trunk.

Districts.  The district in a given community which is served by a single central office is called an office district.  Likewise, the district which is served by a complete exchange is called an exchange district.  An exchange district may, therefore, consist of a number of central-office districts, just as an exchange may comprise a number of central offices.  To illustrate, the entire area served by the exchange of the Chicago Telephone Company in Chicago, embracing the entire city and some of its suburbs, is the Chicago exchange district.  The area served by one of the central offices, such as the Hyde Park office, the Oakland office, the Harrison office, or any of the others, is an office district.

Switchboards.  The apparatus at the central office by which the telephone lines are connected for conversation and afterwards disconnected, and by which the various other functions necessary to the giving of complete telephone service are performed, is called a switchboard.  This may be simple in the case of small exchanges, or of vast complexity in the case of the larger exchanges.

Sometimes the switchboards are of such nature as to require the presence of operators, usually girls, to connect and disconnect the line and perform the other necessary functions, and such switchboards, whether large or small, are termed manual.

Sometimes the switchboards are of such a nature as not to require the presence of operators, the various functions of connection, disconnection, and signaling being performed by the aid of special forms of apparatus which are under the control of the subscriber who makes the call.  Such switchboards are termed automatic.