arlance the telephone
exchange refers not only to the central office and its equipment but to
the lines and instruments connected therewith as well; furthermore, a
telephone exchange may embrace a number of telephone offices that are
interconnected by means of so-called trunk lines for permitting the
communication of subscribers whose lines terminate in one office with
those subscribers whose lines terminate in any other office.

Since a given telephone exchange may contain one or more central
offices, it is proper to distinguish between them by referring to an
exchange which contains but a single central office as a single office
exchange, and to an exchange which contains a plurality of central
offices as a multi-office exchange.

In telephone exchange working, three classes of lines are dealt
with--subscribers' lines, trunk lines, and toll lines.

Subscribers' Lines. The term subscriber is commonly applied to the
patron of the telephone service. His station is, therefore, referred
to as a subscriber's station, and the telephone equipment at any
subscriber's station is referred to as a subscriber's station
equipment. Likewise, a line leading from a central office to one or
more subscribers' stations is called a subscriber's line. A
subscriber's line may, as has been shown in a previous chapter, be an
individual line if it serves but one station, or a party line if it
serves to connect more than one station with the central office.

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