A majority of Bell System subscribers in the mid-20th century in the United States and Canada were serviced by party lines, which carried a billing discount over individual service; during wartime shortages, these were often the only available lines.
British users similarly benefited from the party line discount.
Each party on that single line was identified by a letter, and so to call that party, the exchange would be called and the number asked for would be Gundabluie 1 S for example.
The exchange rang a distinct ring down the Gundabluie 1 line, signalling the party's corresponding letter in Morse code.
In December 1942, University of Tennessee's strategy in an American football game versus University of Mississippi was revealed to the opposing coach as a telephone on the Ole Miss team's bench had been inadvertently wired to the same party line.In 1971, Southern Bell had announced plans for phase-out of party lines in North Carolina.Party lines in the United States were ineligible for Universal Service Fund subsidies and telephone companies converted them to private lines to benefit from the subsidies.This distinctive ring would alert all parties on the line who the call was for.Three short rings signified the call was for the party with the S letter and so on.