In article ,
The Daring Dufas wrote:

Party lines use different ringer frequencies for each customer.

I respectfully disagree.

Two-party service (2FR) signals the "ring party" by delivering ringing
current on the ring side of the pair that passes through a
specially-wired telephone to ground. The "tip party" is signaled by
ringing on the other conductor through a specially-wired set to ground.

4, 6 and 8-party service also uses the above technique on a single pair
but uses distinctive ringing patterns assigned to each party. Example:
Two, short rings.

On an eight-party line, only FOUR parties hear ringing.

OBTrivia: Multi-party service was originally required due to inadequate
infrastructure (not enough pairs). The customers were "bridged" in the
field.

As enough pairs became available, many subscribers re-graded to private
service (1FR, 1FB, etc). Those that kept their two-party service were
bridged in the Central Office. This allowed for easier trouble shooting.

As party line subscribers died off, moved or re-graded to 1FR, their
former partymates enjoyed virtually private service at the lower,
two-party rate.

About once a year, the business office would call these "bridged alone"
subscribers and offer them a private line using Measured Rate service
that was about the same price. About half of these folks would
re-grade. Those that didn't were in for a surprise:

The Central Office Technician would then connect each remaining, bridged
alone subscriber with a new partymate. Most of those would then
promptly call to change to private service.
--

JR