oup. This
selection by the subscriber of a group followed by the automatic
selection from among that group forms the basis of the trunking system.
It is impossible, by means of any simple diagram, to show a complete
scheme of trunking employed, but Fig. 381 will give a fundamental
conception of it. This figure shows how a single calling line, indicated
at the bottom, may find access into any particular line in an office
having a capacity for ten thousand.

=Names of Selecting Switches.= Selecting switches of the "up-and-around"
type are the means by which the calling line selects and connects with
the trunk lines required in building up the connection, and finally
selects and connects with the line of the called subscriber. Where such
a switch is employed for the purpose of selecting a _trunk_, it is
called a selector switch. It is a _first selector_ when it serves to
pick out a major group of lines, _i. e._, a group containing a
particular thousand lines or, in a multi-office system, a group
represented by a complete central office. It is a _second selector_ when
it serves to make the next subdivision of groups; a _third selector_ if
further subdivision of groups is necessary; and finally it is _a
connector_ when it is employed to pick out and connect with the
_particular line in the final group of one hundred lines_ to which the
connection has been brought by the selectors. In a single office of
10,000-line capacity, therefore, we would have first and second
selectors and connectors, the first selectors picking out the thousands,
the second selectors the hundreds, and the connectors the individual
line. In a multi-office system we may have first, second, and third
selectors and connectors, the first selector picking out the office, the
second selector the thousands in that office, the third selector the
hundreds, and the connector the individual lines.

=The Line Switch.= In addition to the selectors and connectors there are
line switches, which are comparatively simple, one individual to each
line. Each of these has the function, purely automatic, of always
connecting a line, as soon as a call is originated on it, to some one of
a smaller group of first selectors available to that line. This idea may
be better grasped when it is understood that, in the earlier systems of
the Automatic Electric Company, there was a first selector permanently
associated with each line. By the addition of the comparatively simple
line