Line marriage is a form of group marriage found in fiction in which the family unit continues to add new spouses of both sexes over time so that the marriage does not end.
Group marriage is sometimes called polygamy or even polygynandry, from a combination of the words polygyny and polyandry. Others suggest that polygynandry and polygamy can only be considered group marriage under certain circumstances.
The Kerista Commune practiced group marriage in San Francisco from 1971 to 1991.
It is difficult to estimate the number of people who actually practice group marriage in modern societies, as this form of marriage is not officially recognized or permitted in any jurisdiction in the U.S., and illegal in many. It is also not always visible when people sharing a residence consider themselves privately to form (or self-identify as) a group marriage. With the legalization of Same-sex marriage in Canada and some parts of the United States, some members of the polyamory movement are talking about a reform movement to also allow group marriage.
The Moon Is a Harsh Mistress describes line families in detail. The characters argue that the line family creates economic continuity and parental stability in an unpredictable, dangerous environment. Manuel's line marriage is said to be over 100 years old. The family is portrayed as economically comfortable because improvements and investments made by previous spouses compounded, rather than being lost between generations. Heinlein also makes it a point that this family is racially diverse. A passing reference to Heinlein's marriage forms is made in David Brin's Infinity's Shore, where a sapient bottlenose dolphin crewmember is noted as belonging to a "line marriage, one of the Heinlein forms."
Group marriages of three partners (called triples) are described as commonplace in the 1966 award-winning novel Babel-17 by Samuel R. Delany. The novel's protagonist, a female starship captain Rydra Wong, once lived in a triple, until one member died and another was put in stasis for an incurable illness. Other crew members, especially those who worked in close three-person teams, are noted for this type of relationship.
Line marriage is also commonly practiced in Joe Haldeman's 1981 novel Worlds. Haldeman describes how individual families joined forces, both in bed and on paper, in order to avoid inheritance taxes. Many of these consensual corporations were made up of three-mate marriages called triunes.
Group marriage is briefly addressed in the 1989 Star Trek novel Star Trek: The Lost Years, by J.M. Dillard (published by Pocket Books). A minor character, Lt. Nguyen, enters into a group marriage and is portrayed as a relatively normal occurrence within the society of the Star Trek world.