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In twentieth-century telephone systems, a party line (also multiparty line or shared service line) was an arrangement in which two or more customers were connected directly to the same local loop. Prior to and during World War II in the United States, party lines were the primary way residential subscribers acquired local telephone service. British users similarly benefited from the party line discount. Farmers in rural Australia used party lines, where a single line travelled miles from the nearest town out to one property then the next.
Telephone companies have offered party lines since the late 1800s, advertising the service in newspapers of general circulation since at least 1897, although subscribers in all but the most rural areas have usually been offered the option to upgrade to private line at an additional monthly charge.
Party lines were widely used in Australia where sparsely populated remote properties are spread across large distances. These were operated by the Government Post Master General department.
In rural areas in the early twentieth century, additional subscribers and telephones were frequently connected to the single loop available, with numbers reaching into the several dozen, leading to extreme congestion. The completely non-private party lines were a cultural fixture of rural areas for many decades, and were frequently used as a source of entertainment and gossip, as well as a means of quickly alerting entire neighborhoods in case of emergencies such as fires, as written in The Thorn Birds by Colleen McCullough.
In both world wars, rationing of strategic metals such as copper led to telephone companies connecting multiple subscribers to each line as a means to avoid the need for new individual local loops. This often led to congestion. Nearly three-quarters of Pennsylvania residential service in 1943 was party line, with users encouraged to limit calls to five minutes. Shortages persisted for a few years after each war; private lines in Montréal remained in short supply at the end of 1919, a year after the end of the Great War, and similar shortages were reported by telephone companies in Florida as late as 1948, three years after the end of World War II. Some rural users had to run their own wires to reach the utility's lines.
In December 1942, University of Tennessee's strategy in a US 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 May 1952, an alleged bookmaking operation in St. Petersburg, Florida was shut down after one month of operation in a rented storefront using a party line telephone. In June 1968, the conviction of three Winter Park, Florida men on bookmaking charges was overturned as police had used a party line telephone in a rented house on the same line as the suspects to unlawfully intercept their communications.
Technological attempts to prevent party line calls being audible to every subscriber on the line began in the 1920s, becoming viable with the initial tests of transistorised pair gain devices in 1955.
Many areas have laws requiring a person engaged in a call on a party line to end the call immediately if another party needs the line for an emergency. These laws additionally provide penalties for falsifying an emergency situation to the parties in an existing telephone call, in order to gain access to a party line.
In May 1955, a Poughkeepsie, New York woman was convicted by a grand jury after her refusal to relinquish a party line delayed a volunteer firefighter's effort to report a grass fire; the fire destroyed a shed and a barn. She was given a suspended sentence. In June 1970, a sixteen-year old girl and a woman were charged after refusing to relinquish a party line to allow a distress call as three boys drowned in a pond in Walsenburg, Colorado.
Objections about one party monopolizing a multi-party line were the staple of complaints to telephone companies and letters to advice columnists for many years and eavesdropping on calls remained an ongoing concern.
By the 1980s, party lines were dying in most localities as they could not support subscriber-owned equipment such as answering machines and computer modems; the mechanical switching equipment required to make them operate was also being supplanted by electronic switching systems and later digital switches. The new telephone exchange equipment offered optional features such as call forwarding and call waiting which were profitable to telephone companies but was designed for private, not party, lines.
In 1971, Southern Bell had announced its first plans to begin phase-out of party lines in North Carolina. In 1989, the Chesapeake and Potomac Telephone Company replaced party line with private line in Talcott, West Virginia, a rural area which once had as many as sixteen subscribers on one line. In 1991, Southwestern Bell set out to replace all of its party lines in Texas with private line by 1995. Woodbury, Connecticut's independent telephone company abandoned its last party lines in 1991, the last in that state to do so.
One of the reasons party lines stopped being used in the United States was that they were ineligible for Universal Service Fund subsidies; telephone companies converted them to private lines quickly once they became ineligible. Universities also began to phase out the systems, which were once common in student dormitories. Illinois State University hung up its last party line in 1990.
One of the last manual telephone exchanges which still used party lines in Australia was closed down in 1986 at the township of Collarenebri, New South Wales. In that town, most town residents had a telephone number of only three digits, and to make a call outside the exchange area it was necessary to call the exchange to place a call. For rural residents, many were on a single telephone line identified by a number and a property name, for example one party line was called Gundabluie 1 line. 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. 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.
Originally, in order to distinguish one line subscriber from another, operators developed different ringing cadences for the subscribers, so that if the call was for the first subscriber to the line, the ring would follow one pattern such as two short rings, if the call was for the second subscriber, the ring would sound another way, such as a short ring followed by a long one, and so on. Since all parties utilized the same line, it was possible for subscribers to listen in on other subscribers' calls. Frequently ringing phones were an annoyance, so selective ringing methods were introduced in the mid-twentieth century.
Especially effective on two-party lines was the distinction between tip party and ring party. Each telephone bell, rather than being connected across tip and ring as usual, was connected from one wire to local ground. Thus only the selected station in a two party line would ring. For multiparty lines all the "tip parties" or all the "ring parties" would ring, in this semi-selective scheme. This system was also used in the United Kingdom where X and Y subscribers on an A&B wire system would be rung on B wire and earth for the X subscriber and on the A wire to earth in the case of the Y subscriber. The momentary earth condition to initiate a call by first getting dial tone would have a similar convention. Frequently, only sets that are owned and installed by the connecting telephone company are permitted on the party line; the phone's bell had to be configured and grounded by a technician for the appropriate ringing signal. An off-the-shelf telephone may ring for both lines if plugged into the party line.
Later, independent systems applied multiple ringing frequencies for fully selective ringing. (The Bell System eschewed frequency selective ringing.) The ringers in party-line phones were tuned to distinguish several different ringing signals so that only the desired party's phone would actually ring. In this arrangement the only inconvenience of a party line was occasionally finding the line in use (by hearing talking) when one picked up the phone to make a call. If one of the parties used the phone heavily, then the inconvenience for the others was more than occasional, as depicted in the 1959 comedy film Pillow Talk.
Even for lines with selective ringing, calls to another party on the same party line, known as "reverting calls", required special equipment and procedures. One such piece of equipment allowed a user to hear the conversation on the line without interrupting the conversation.
Party line usage was at one time common on railways, where numerous telephones were connected to a single pair of wires. Usually a long ring of many turns of the handle would alert the exchange that a connection was required to another destination. The problem of selective calling was also solved by a mechanical device which could selectively ring one or a group of stations.
Modern telephone systems do in some cases use one copper pair as a shared medium on which to carry telephone service to multiple subscribers in a distant location. Various pair gain schemes allow time division multiplexing or frequency division multiplexing so that multiple calls are carried simultaneously on the same pair without interference. A distant suburb may have a subscriber loop carrier or digital loop carrier system in which cabinetised equipment is located near the subscribers to connect the multiple, individual phones to one common line back to a central office exchange. A single optical fibre can also be shared between multiple subscribers in fibre to the cabinet systems.
Digital wireless connections (such as cellular telephones or voice-over-IP running over rural wireless Internet) are also inherently a shared medium. A sufficiently high usage of simultaneous active connections will cause a mobile telephone network to become congested or voice transmitted by Internet telephony to break up unintelligibly. Operated within their capacity, these systems do give the illusion of a virtual private line as calls may be carried without mutual interference.
Party lines remain primarily in rural areas where local loops are long and private loops uneconomical when spread sparsely over a large area. Privacy is limited and congestion often occurs. In isolated communities, party lines have been used without any direct connection to the outside world.
One example of a community linked by party line is in Big Santa Anita Canyon high in the mountains above Los Angeles, near Sierra Madre, California, where 81 cabins, a group camp and a pack station all communicate by magneto-type crank phones. One ring is for the pack station, two rings for the camp and three rings means all cabins pick up.
In modern use, the term "party line" has occasionally been used to market conference bridge and voice bulletin board service, but these are not party lines in the original sense of the term as users call in using multiple, individual lines.