he connections are made between the various lines
by means of pairs of cords and plugs. The private branch-exchange trunk
lines usually terminate on the private branch board in jacks but in some
cases plug-ended trunks are used.

[Illustration: Fig. 439. Key Type, Private Branch Board]

The line signals may consist in mechanical visual signals or in lamps,
the choice between these depending largely on the source of battery
supply at the branch exchange, a matter which will be considered later.
The trunk-line signals at the private branch board are usually ordinary
drops which are thrown when the main-exchange operator rings on the line
as she would on an ordinary subscriber's line. Frequently, however, lamp
signals are used for this purpose, being operated by locking relays
energized when the main-office operator rings or, in some cases,
operated at the time when the main-office operator plugs into the
trunk-line jack.

[Illustration: Fig. 440. Circuits, Key-Type Board]

_Key Type._ For small private branch-exchange switchboards, a type
employing no cords and plugs has come into great favor during recent
years. Instead of connecting the lines by jacks and plugs, they are
connected by means of keys closely resembling ordinary ringing and
listening keys. Such a switchboard is shown in Fig. 439, this having a
capacity of three trunks, seven local lines, and the equivalent of five
cord circuits. The drops associated with the three trunks may be seen in
the upper left-hand side of the face of the switchboard. Immediately
below these in three vertical rows are the keys which are used in
connecting the trunks with the "cord circuits" or connecting bus wires.
At the right of the drop associated with the trunks are seven visual
signals, these being the calling signals of the local lines. The seven
vertical rows of keys, immediately to the right of the three trunk-line
rows, are the line keys. The throwing of any one of these keys and of a
trunk-line key in the same horizontal row in the same direction will
connect a line with a trunk through the corresponding bus wires, leaving
one of the supervisory visual signals, shown at the extreme top of the
board, connected with the circuit. The keys in a single row at the right
are those by means of which the operator may bridge her talking set
across any of the "cord circuits." The circuits of this particular board
are shown in Fig. 440. This is equipped for common-battery working, t