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Trope · Genre
Related: OT4, Poly
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Sedoretu is a poly marriage arrangement invented by Ursula K. Le Guin in her science fiction story "A Fisherman of the Inland Sea".


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?[1]

Rise In Popularity

The first major fanwork to use the idea of a sedoretu is An Ever-Fixed Mark in the Merlin fandom, written in 2009.

melannen also wrote a popular piece of meta outlining various types of plural marriage, including sedoretu. She comments:

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.[2]

Notable Works


  1. Ursula K. Le Guin. The Birthday of the World - Excerpts: Mountain Ways
  2. Can't have one without the other Accessed 12 April 2011.