form, it is a line joining a telephone to some means of connecting it
at will to another.

A telephone exchange central office contains means for connecting
lines at will in that useful way. The least complicated machine for
that purpose is a switchboard to be operated by hand, having some way
of letting the operator know that a connection is wished and a way of
making it. The customary way of connecting the lines always has been
by means of flexible conductors fitted with plugs to be inserted in
sockets. If the switchboard be small enough so that all the lines are
within arm's reach of the operator, the whole process is individual,
and may be said to be at its best and simplest. There are but few
communities, however, in which the number of lines to be served and
calls to be answered is small enough so that the entire traffic of the
exchange can be handled by a single person. An obvious way, therefore,
is to provide as many operators in a central office as may be required
by the number of calls to be answered, and to terminate before each of
the operators enough of the lines to bring enough work to keep that
operator economically occupied. This presents the additional problem,
how to connect a line terminating before one operator to a line
normally terminating before another operator. The obvious answer is to
provide lines from each operator's place of work to each other
operator's place, connecting a calling line to some one of these lines
which are local within the central office, and, in turn, connecting
that chosen local line to the line which is called.

Such lines between operators have come to be known as _trunk lines_,
because of the obvious analogy to trunk lines of railways between
common centers, and such a system of telephone lines may be called a
_trunking system_. Very good service has been given and can be given