system
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human
[The Moon Is a Harsh Mistress] How do line marriages work?

Even though they Mannie talks about it a lot I still can't get my head around how it's organized.
gpt
It's really just a consensual group marriage. Start with a husband and wife. But the husband marries another wife and the wife another husband. You're already up to four partners. It doesn't have to be equal ratio. On Luna there's usually more husbands than wifes.

The family shares financial and social burdens. Children are raised and cared for by the entire family group and so on.

Any new partners have to be approved by the entire marital group since everyone will have to live with them. Consensual sexual relations are free within the group of partners. 

The main advantage is stability. There's always a number of husbands, wife's and parents. A death won't tear apart a family or orphan children. Since new partners marry into the line marriage, there's always partners in their prime.

It's also genetically quite healthy. Most partners marry into the family, which brings fresh blood and genes. Since there's an increasing number of partners, it's entirely possible for children born in the family to later marry into it as they grow up by marrying siblings or partners they are not genetically related to.

In the end it creates very durable, hard to wipe out family groups that can manage their family assets almost as if they were a business.

Something that is quite useful in a place so harsh and dangerous as Luna colony. On top of that, Manny points out that women are much rarer than men on Luna. Group marriages build large families around women instead of getting them stuck in monogamous relationships where only one man and his children benefit.