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 class of manual switchboards called the toll board of which it will be necessary to treat.  Telephone calls made by one person for another within the limits of the same exchange district are usually charged for either by a flat rate per month, or by a certain charge for each call.  This is usually regardless of the duration of the conversation following the call.  On the other hand, where a call is made by one party for another outside of the limits of the exchange district and, therefore, in some other exchange district, a charge is usually made, based on the time that the connecting long-distance line is employed.  Such calls and their ensuing conversations are charged for at a very much higher rate than the purely local calls, this rate depending on the distance between the stations involved.  The making up of connections between a long-distance and a local line is usually done by means of operators other than those employed in handling the local calls, who work either by means of special equipment located on the local board, or by means of a separate board.  Such equipments for handling long-distance or toll traffic are commonly termed toll switchboards.