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This trope involves any fanwork where a relationship exists based on the following rules:
- Note for readers unfamiliar with the planet O:
- Ki'O society is divided into two halves or moieties, called (for ancient religious reasons) the Morning and the Evening. You belong to your mother's moiety, and you can't have sex with anybody of your moiety.
- Marriage on O is a foursome, the sedoretu — a man and a woman from the Morning moiety and a man and a woman from the Evening moiety. You're expected to have sex with both your spouses of the other moiety, and not to have sex with your spouse of your own moiety. So each sedoretu has two expected heterosexual relationships, two expected homosexual relationships, and two forbidden heterosexual relationships.
- The expected relationships within each sedoretu are:
- The Morning woman and the Evening man (the "Morning marriage")
- The Evening woman and the Morning man (the "Evening marriage")
- The Morning woman and the Evening woman (the "Day marriage")
- The Morning man and the Evening man (the "Night marriage")
- The forbidden relationships are between the Morning woman and the Morning man, and between the Evening woman and the Evening man, and they aren't called anything, except sacrilege.
- It's just as complicated as it sounds, but aren't most marriages?
Additional Rules & Terminology
One's parents are mother and father. The other couple in the sedoretu are one's Othermother and Otherfather. The "baby word" for othermother is "ombu".
People can attach themselves to a sibling's marriage as aunts/uncles, which permits them to have culturally-permissable sex with the members of the marriage of the opposite moiety. The children of these relationships are considered "cousins" to the children of the marriage.
As a sedoretu is 4 people, it can expand to 8 places, with each of the people in the marriage bringing in a sibling(s) as aunts or uncles.
Children of the same pair in a marriage are considered "siblings" to each other, and are "germanes" to the children of the other marriage. Children of an aunt/uncle is considered a cousin to the other children.
Brothers, sisters, and "first cousins" (which appears from context in the text to refer to children of the same mother by different fathers) may not marry each other, as they all share a moiety.
There are regional differences in marriage customs pertaining to germanes. In some regions, germanes may marriage, and in other regions, that is considered an unacceptable marriage. 
Alternatives to sedoretu relationships within the sedoretu canon
At the Centers are "untraditional" relationships, which includes same-moiety relationships.
Rise In Popularity
Sedoretu are amazing fun, and there need to be for fandom AUs that use them: I only know of the one that's linked above, in BBC Merlin fandom. But there are so many other fandoms that have central foursomes for whom a sedoretu would be perfect! To start with I really want to see some Stargate teams sent to O and assumed to be sedoretu. But there are plenty more possibilities. To bring up Harry Potter again: Harry and Hermione are both Evening people, obviously, and Ron and Ginny are both Morning since Molly obvs. is, so they can all get married in a sedoretu! And Harry/Ginny and Ron/Hermione can be awesome, and so can Ron/Harry and Ginny/Hermione, and meanwhile Ron and Ginny can be all sibling-y together and Harry and Hermione can be total best friends.
Really in any situation where people solve the problem of slash + canon het by pairing up the het love interests too becomes an amazing sedoretu. This happens especially a lot in RPS fandoms but there are lots of fandoms where sedoretu could fit seamlessly. (Hodgins and Bones are Evening, Booth and Angela are Morning: y/y/mfy? And Wilson needs to find a nice Evening girl who wants to stick around, so that he and Cuddy and House can get married already.)
This then inspired more meta and comment fic.
- An Ever-Fixed Mark by imperfectcircle
- Buffy of the Ki'O by sapote
- Doctor Who comment fic by recessional
- Tony Stark is Not Getting Married (Today) by Circ_Bamboo (Avengers (Marvel))
- Just As Complicated As It Sounds by Mific (Stargate Atlantis)
Archives & Fannish Links
- Sedoretu tag on AO3
- Morning Evening Day Night: A Sedoretu Fanworks Collection on AO3
- Sedoretu Fic Fest, including lots and lots of unused prompts
- Ursula K. Le Guin. The Birthday of the World - Excerpts: Mountain Ways
- Fisherman of the inland sea, page 165
- Fisherman of the inland sea page 167
- Fisherman of the inland sea page 184
- Can't have one without the other Accessed 12 April 2011.