lication of this
power to the various switching mechanisms. These details will be more
fully dealt with in subsequent chapters.

_Multiple vs. Trunking._ It has been shown in the preceding portion of
this work that the tendency in manual switchboard practice has been away
from trunking between the various sections or positions of a board, and
toward the multiple idea of operating, wherein each operator is able to
complete the connection with any line in the same office without
resorting to trunks or to the aid of other operators. Strangely enough
the reverse has been true in the development of the automatic system. As
long as the inventors tried to follow the most successful practice in
manual working, failure resulted. The automatic systems of today are
essentially trunking systems and while they all involve multiple
connections in greater or less degree, all of them depend fundamentally
upon the extending of the calling line by separate lengths until it
finally reaches and connects with the called line.

_Grouping of Subscribers._ In this connection we wish to point out here
two very essential features without which, so far as we are aware, no
automatic telephone system has been able to operate successfully. The
first of these is the division of the total number of lines in any
office of the exchange into comparatively small groups and the
employment of correspondingly small switch units for each group. Many of
the early automatic systems that were proposed involved the idea of
having each switch capable in itself of making connection with any line
in the entire office. As long as the number of lines was small--one
hundred or thereabouts--this might be all right, but where the lines
number in the thousands, it is readily seen that the switches would be
of prohibitive size and cost.

_Trunking between Groups._ This feature made necessary the employment of
trunk connections between groups. By means of these the lines are
extended a step at a time, first entering a large group of groups,
containing the desired subscriber; then entering the smaller group
containing that subscriber; and lastly entering into connection with the
line itself. The carrying out of this idea was greatly complicated by
the necessity of providing for many simultaneous connections through the
switchboard. It was comparatively easy to accomplish the extension of
one line through a series of links or trunks to another line, but it
was not so easy