The larger majority of human intimate relationships have involved two people, historically a man and a woman (though [[UsefulNotes/{{Homosexual}} not always]]): this man and woman live together, have children together, have sex with each other, and don't turn to other people to fulfill any of those needs. However, it is possible to put together intimate relationships that involve more than two people. These situations are what this article is about.

The practice of having more than one spouse is called ''polygamy''; the conditions of having more than one husband or wife actually have their own names (''polyandry'' and ''polygyny'' respectively).

* '''Polyandry''': One wife, and two or more husbands
** The advantages: In a sexist world where men work and women stay at home, is that her children receive more care and nutrition; the presence of more than one breadwinner increases the likelihood that the little tots will live to adulthood--an important consideration if, as it was throughout history, child mortality rates are in the 50% area.
** Another major factor is that in mountainous regions where land is scarce, if brothers marry the same women, their joint household can inherit the family land together. Otherwise, the land would either (a) be split into pieces that are too small to support any of the brothers, or (b) it would all be left to the eldest brothers, with the others getting nothing.
** Alternatively, at varying times in varying societies, traditional men's work--such as farm labor was vital for the family. In other words, multiple husbands meant less work for each of the men and greater family productivity over all.
* '''Polygyny''': One husband, and two or more wives
** The advantages: (in the same sexist world where men are the breadwinners) It allows women to get very high quality husbands. Well, think of it this way: if you could contrive it that your child would be fathered by Creator/BradPitt, or UsefulNotes/AlbertEinstein, or Music/JohannSebastianBach, but to do it you had to become that man's second wife, would you? You might: the benefits to your child outweigh the inconvenience to you. Besides, these are rich, successful men who can probably provide for you more healthily than their currently-single competitors. This is the impetus behind animal herds involving one alpha male and a bunch of women: he who is best and most fit (as chosen by natural selection) is the one I want my kids to be fathered by, for the sake of their futures.
** In a society where men are warriors or soldiers, they get killed at a higher rate then women, leaving a gender imbalance. Doubling up on husbands then becomes the only way for there to be enough to go around. Also, in the widows of the dead warriors are now single mothers, and--in the same sexist world where men are the breadwinners--that's a very hard position for her and her children. If their society allows her to remarry, it's better for her kids.
** Alternatively, at varying times in varying societies, traditional women's work that was the most labor intensive or provided the most economic advantage for the family--for instance, some native American tribes went from monogamy to polygyny as American colonization became more prominent, because it became financially advantageous for the families to be able to produce more textiles and other goods that the women typically made, for trade and the like. In other words, multiple wives meant less work for each of the women and greater family productivity over all.
* '''Group marriage''': And finally there's the group marriage, which is a marriage between three or more people of any combination of sexes and genders. These can get complicated, but in theory bring all the advantages listed above under one (very big) roof.

If you're looking at these ideas and thinking that they're all about straight practicality, well, you're right. The idea of marrying for love--a marriage in which your personal happiness is of prime importance over questions of procreation, economics, etc--is much NewerThanTheyThink; it's only been standard practice since about the 1600s.
Before then, marriages had more to do with child-rearing and political alliances than anything else. And even today, would you seriously marry someone who you thought would make a bad parent? Or, for that matter, who couldn't provide for you if push comes to shove? You probably wouldn't.

Incidentally, polygyny was legal in several ancient cultures, including the ancient Israelites of Literature/TheBible. (Solomon was said to have 700 wives.) Polyandry... not so much, and has mostly been practiced in the Himalayas.
The next idea down the list (from most commitment to least) is the idea of ''polyamory'', which is when you are in more than one committed relationship at a time, with the consent of everyone involved. This is a modern variant, focusing on love and dating.

Most people will tell you that, even if you can love more than one person at a time, it's hard to be ''committed'' to more than one person at a time, due to the selfish aspects of human nature. Polyamorists disagree. They don't reject commitment, but they do reject exclusivity, jealousy, possessiveness and the negative or limiting emotions that seem to come with it so very often. The main difference between this and polygamy is that marriage is not considered a necessary part of a polyamorous relationship--and nor, for that matter, is sex. Polyamory simply means that you want to form significant emotional bonds--of any manner--with more than one person. In that sense you could argue that we are ''all'' polyamorists: even people who get married and have a spouse (or two) still have emotional bonds with their friends, their siblings, their parents. Not the same ''kinds'' of bonds they have with their spouse, [[ParentalIncest or so we hope]], but bonds nonetheless.
You also have ''swinging'', which is the belief that you can be in a committed emotional relationship while still having sex with other people--with, of course, the consent of everyone involved.

The root of swinging is the understanding that sex and love are not faces of the same coin, and that one does not have to proceed directly from the other. This is of course patently TruthInTelevision; regardless of what the "SexEqualsLove" trope would have you believe, it's possible to have sex with someone you don't love (see: casual sex, RapeTropes), and possible to love someone romantically without having sex with them (see: CourtlyLove, ChastityCouple). People who swing simply maintain this idea into their committed relationships: as long as they have permission from their spouse[=/=]significant other[=/=]etc and are taking all appropriate precautions, they don't see anything wrong with having some casual sex on the side. Obviously, the spouse gets the same rights and privileges as you do.
And then finally down at the bottom is the idea of the two-person committed monogamous relationship, which is what Western culture ([[FreeLoveFuture currently]]) considers the ideal. In this relationship it's against the rules to do anything sexual or romantic with someone besides your spouse[=/=]significant other[=/=]etc. If you break the rules, you're cheating. If you ''change'' the rules, then you're not in a monogamous relationship anymore; you belong to one of the poly* categories discussed already.
!Consent of partners
You'll notice that none of these relationships give you the right to just go out and do whatever (or whomever) you want, without the permission or consent of your partners. Generally, that's reserved for being single. All these relationships involve being committed to somebody--sometimes more than one somebody--and if you get together with someone when your partner(s) has told you not to, then you are cheating and that's that. This is where multi-partner relationships get tricky. Say you're in a relationship with Alice and Bob, and you then want to sleep with Charlie on the side. Alice is amenable... But Bob says no, and won't budge. (Maybe Charlie was mean to them in high school.) It's hard enough to get permission to do anything from ''one'' spouse; imagine having to clear your actions with two! This is one of the arguments people bring up when declaring that multi-partner relationships don't work... and, to be fair, they've got a point. But just because something is difficult doesn't mean it's impossible.
!Open vs closed relationship
This brings us to one last distinction: an ''open'' relationship versus a ''closed'' one. A closed relationship is just that: whoever you're with, that person[=/=]those people are it for you.

It's possible to have a closed polygamous marriage--for instance: say you have three wives, and all three of them have you. If the marriage is closed, then that's it as far as sex, intimacy, etc is concerned: you're not allowed to get any more wives (or sleep with anyone else on the side), and your three wives are likewise limited to you (and maybe each other).

In an open relationship, partners are allowed or even encouraged to venture outside their current roster of (sexual and/or romantic) partners. If you're a swinger, that line starts to blur a little: sex can be a lot more casual, although it may still involve deep romantic feelings, or great friendships with various sex partners; and all of it can change for individuals over time, of course. But it's still possible to swing with only a specific set of people ("Okay, hon, we're going to sleep with each other and also with the Joneses, but with nobody else.") or for limits to be placed on some other element ("If you find anyone you'd enjoy spending the night with, hun, have fun! Just make sure they know it's not going to go beyond sex.")

Long story short, a "closed" relationship is one that is exclusive[[note]]This article, back when it was written solely by someone who is admittedly mono*, used the word "faithful" here. This word can be offensive to poly* individuals, as the word itself strongly implies cheating, which would be ''nonconsensual'' to at least one other partner.[[/note]]. The only question remaining is ''who'' you're being exclusive with.
Finally, we need to talk about '''death''' a little bit here, because there are cultural traditions in which widows and widowers are considered to be still married to their dead spouses. In some traditions, marriages are regarded as being dissolved upon death, so that only ''living'' people can be considered married to each other. In others, marriages still exist in an afterlife of some sort, whether or not they actually practice polygamy between living people.[[note]]For example, though the Church of Jesus Christ of Latter-day Saints (the largest denomination of UsefulNotes/{{Mormonism}}) has long since prohibited polygamy among the living, they recognize ''posthumous'' polygamy between the dead and between the living and dead, meaning that a living person remains married to a deceased spouse and is free to remarry a new living spouse, who will ''also'' remain married to them when deceased. Then when the widow or widower themselves dies, everyone they permanently married is considered to still be married to them in the afterlife.[[/note]] Heck, some people don't even believe in divorce!--Catholicism, the biggest branch of UsefulNotes/{{Christianity}}, subscribes to the idea that if two people get married, they are spiritually linked ''forever'', even if they both decide they don't want to be anymore. Catholicism also exemplifies the uncertainty about widowed spouses re-marrying, with one influential writer, [[ Tertullian]], arguing both for ''and'' against it at different times. Needless to say, marriage customs can be very complicated in regards to these things.
!Personal opinion
Finally, there is room for a fair amount of personal discomfort in these things. "I don't believe that these things could actually work," you say to yourself. "I mean, sure, they ''think'' it works, and they're certainly trying hard, but it'll never hold together." In the end, all you can do is remind yourself that one man's {{Fetish}} is another man's {{Squick}}. Yes, there are people out there who think that anything besides a committed two-person relationship is preposterous. Likewise, there are people who would laugh at the idea of monogamy! "We all want variety in our sex lives," they would say, "and unless you have a ''very'' understanding partner, you're not likely to get that variety from just him or her. Do both of you a favor and get permission to play around." They too have a valid argument. Find out what works for you.