oard itself, or a failure of
its sources of current, or an accident to the cable approaches, the
single office practice is like putting all one's eggs in one basket.

Another factor that has contributed to the adoption of smaller
switchboard capacities is the fact that in the very large cities even a
40,000 line multiple switchboard would still not remove the necessity of
multi-office exchanges with the consequent certainty that a large
proportion of the calls would have to be trunked anyway.

Undoubtedly, one of the reasons for the difference between American and
European practice is the better results that American operating
companies have been able to secure in the handling of calls at the
incoming end of trunks. This is due, no doubt, in part to the
differences in social and economic conditions under which exchanges are
operated in this country and abroad, and also in part to the
characteristics of the English tongue when compared to some of the other
tongues in the matter of ease with which numbers may be spoken. In
America it has been found possible to so perfect the operation of
trunking under proper operating conditions and with good equipment as to
relieve multi-office practice of many of the disadvantages which have
been urged against it.

=Classification.= Broadly speaking there are two general methods that
may be employed in trunking between exchanges. The first and simplest of
these methods is to employ so-called _two-way trunks_. These, as their
name indicates, may be used for completing connections between offices
in either direction, that is, whether the call originates at one end or
the other. The other way is by the use of _one-way trunks_, wherein each
trunk carries traffic in one direction only. Where such is the case, one
end of the trunk is always used for connecting with the calling
subscriber's line and is termed the _outgoing_ end, and the other end is
always used in completing the connection with the called subscriber's
line, and is referred to as the _incoming_ end. Traffic in the other
direction is handled by another set of trunks differing from the first
set only in that their outgoing and incoming ends are reversed.

As has already been pointed out, a system of trunks employing two-way
trunks is called a _single-track system_, and a system involving two
sets of one-way trunks is called a _double-track system_. It is to be
noted that the terms outgoing and incoming, as applied to the ends