may seem.

A telephone line that is connected with an exchange is an exchange
line, and it is a party line if it has more than one station on it. It
is an individual line or a single party line if it has but a single
station on it. A line which has no central-office connection is called
an "isolated line," and it is a party line if it has more than two
stations on it.

The problem of mere speech transmission on party lines is comparatively
easy, being scarcely more complex than that involved in private or
single party lines. This is not true, however, of the problem of
signaling the various stations. This is because the line is for the
common use of all its patrons or subscribers, as they are termed, and
the necessity therefore exists that the person sending a signal, whether
operator or subscriber, shall be able in some way to inform a person at
the desired station that the call is intended for that station. There
are two general ways of accomplishing this purpose.

(_1_) The first and simplest of these ways is to make no provision for
ringing any one bell on the line to the exclusion of the others, and
thus allow all bells to ring at once whenever any station on the line
is wanted. Where this is done, in order to prevent all stations from
answering, it is necessary, in some way, to convey to the desired
station the information that the call is intended for that station,
and to all of the other stations the information that the call is not
intended for them. This is done on such lines by what is called "code
ringing," the code consisting of various combinations of long and
short rings.

(_2_) The other and more complex way is to arrange for selective
ringing, so that the person sending the call may ring the bell at the
station desired, allowing the bells at all the other stations to
remain quiet.

[Illustration: Fig. 163. Grounded-Circuit Series Line]

These two general classes of party-line systems may, therefore, be
termed "non-selective" and "selective" systems. Non-selective party
lines are largely used both on lines having connection with a central
office, and through the central office the privilege of connection
with other lines, and on isolated lines having no central-office
connection. The greatest field of usefulness of non-selective lines is
in rural districts and in connection with exchanges in serving rather
sparsely settled districts where the cost of individual lines or even
lines serving but a