A fascinating article was posted a few minutes ago to the comp.os.linux.ubuntu newsgroup off-topic in a thread about computer motherboards by John F. Morse.
I don't recall seeing any party-line-related articles here in 10+ years and it doesn't appear John F. Morse has ever posted here (per a Google search), so you might find this interesting, too.
Copy'n'pasted article follows:
Those 4-party lines used a small three-wire gas vacuum tube in the phones. It wasn't a "triode" but actually a gas-fired (ignited) diode. It was wired differently in the phones to pass superimposed ringing current depending on whether the party was #1 or #3. The party #2 and #4 simply used the opposite side of the line (the return in both cases was ground).
Party #1 and #3 were wired ring-to-ground, while party #2 and #4 were wired tip-to-ground.
Tip and ring are terms from the old manual switchboard days, with tip having a positive polarity and ring a negative, usually ~48 VDC. The ringing current was 85-130 VAC, 20 Hertz (usually, but read on) superimposed on the ~48 VDC.
Tip was the green wire, and ring was red. If colors were not used, then ring had a ridge on the side of a cable, like a drop wire, or was on the right when terminals were side-by-side, or behind (rear of) the tip terminal. Memory aid is all "R"s: Ring-Right-Ridge-Rear.
When terminals were one above another, then it was "Tip-Top."
Two-party lines didn't require the tube. Single party lines didn't ring to ground, but across the line, tip-to-ring. The ringers in each phone set were therefore wired to respond to the proper party position.
In most central offices, the numbers ending in 000 through 499 had one superimposed polarity, while numbers 500-999 had the opposite. This allowed 4-party lines to ring without anyone hearing the other party ringing. However, it made moving party "fills" difficult without changing someone's number ("lines" and "numbers" are not associated together). These "fills" were made to irritate people, so they wouldn't ride along for free on a lightly-loaded multi-party line.
The 8-party lines (and more) used the old long and short ringing signals, and were in the old manual switchboard days, usually for rural areas where open wire lines would run for miles.
Those 10-party lines in Alaska (and many other non-Bell areas), used either a harmonic ringer or a Decimonic ringer.
The harmonic ringers responded to different ringing frequencies. They had different weights on the clapper arm which acted like a tuning fork. They would only respond to a narrow ringing frequency, which were something like 8 Hz, 16 Hz, 24 Hz, 33 Hz, 41 Hz. Back then we didn't have Hz (Hertz), but CPS (Cycles per Second).
The Decimonic ringers used "decimal" frequencies like 10 Hz, 20 Hz, 30 Hz, 40 Hz, 50 Hz.
Both of these harmonic schemes were available in automatic dial central offices. By ringing tip-to-ground and ring-to-ground, these five frequencies could serve 10 different telephones on one line.
One of the most stupid things I saw when I was installing phones for Southwestern Bell Telephone Company around 1969, was when I went to a new customer's house to install a 2-party service. This elderly guy was a roommate of another old dude who already had a 2-party line in that house.
You guessed it! The assignment center assigned the new customer to the same line, so I really had no real wiring to do at their house. Just connect the second phone reversed from the first one.
Yes, they each had their own "private" ringing, but only one of them could use their phone at the same time, which is how any party line works. The oddity is they each paid about 70% of what a "straight" line would have cost, and only gained separate ringing.
Since they lived in the same house, I though it would be easier, and certainly cheaper, for any caller to simply ask for the opposite roommate if they wanted them.
Instead of paying a 140% telephone bill.