The telephone party line was just a local loop telephone circuit that was shared by everyone tied into that local loop telephone service.
There would be a distinctive ringing that would be for certain people so they would know if that was there ring and someone was calling them.
The telephone party line used mechanical switching systems which recognized certain codes for revertive calls; these no longer work on modern electronic or digital switch gear.
The year of 1991 is when telephone party lines ended completely.
Although the telephone party lines became less popular and less in use before 1991 it wasn't until the year of 1991 that the telephone party line was no longer used anymore.
By the year 1991 most people had a private telephone line because the telephone service had been able to string a lot more lines.
Telephone party lines were used before private telephone lines because it was cheaper to install less telephone lines and allowed people to have use of the telephone to make calls without needing a private line.
However telephone party lines offered no privacy as anyone who had a telephone on the party line could pick up their phone and listen into your call.
Telephone party lines were a source of entertainment and gossip and it was hard to make calls on them because the telephone could be tied up.
The telephone party line was popular in the 30's and 40's and 50's but became less popular in the 70's.