from
the subscribers whose lines terminate in that office. In the case of
calls for lines in that same office, they complete the connection
themselves without the assistance of the other operators. On the other
hand, the calls for lines in another office are handled through trunk
lines leading to that other office, as before described, and these
trunk lines always terminate in the _B_ board at that office. The _B_
operators are, therefore, those operators who receive the calls over
trunk lines and complete the connection with the line of the
subscriber desired.

To define these terms more specifically, an _A_ board is a multiple
switchboard in which the subscriber's lines of a given office district
terminate. For this reason the _A_ board is frequently referred to as
a subscribers' board, and the operators who work at these boards and
who answer the calls of the subscribers are called _A_ operators or
subscribers' operators. _B_ boards are switchboards in which terminate
the incoming ends of the trunk lines leading from other offices in the
same exchange. These boards are frequently called incoming trunk
boards, or merely trunk boards, and the operators who work at them and
who receive the directions from the _A_ operators at the other boards
are called _B_ operators, or incoming trunk operators.

The circuits which are confined wholly to the use of operators and
over which the instructions from one operator to another are sent, as
in the case of the _A_ operator giving an order for a connection to a
_B_ operator at another switchboard, are designated _call circuits_ or
_order wire circuits_.

Sometimes trunk lines are so arranged that connections may be
originated at either of their ends. In other cases they are so arranged
that one group of trunk lines connecting two offices is for the traffic
in one direction only, while another group leading between the same two
offices is for handling only the traffic in the other direction. Trunk
lines are called _one-way_ or _two-way_ trunks, according to whether
they handle the traffic in one direction or in two. A trunking system,
where the same trunks handle traffic both ways, is called a
_single-track system_; and, on the other hand, a system in which there
are two groups of trunks, one handling traffic in one direction and the
other in the other, is called a _double-track system_. This
nomenclature is obviously borrowed from railroad practice.

There is still another cl