A type of LoveTriangle -- heavy on the triangle, light on the love. When a girlfriend leaves her boyfriend, and ends up in a relationship with the boyfriend's ideological [[TheRival rival]], resulting in a vendetta between the two men.

It works with the genders switched around, too.

Compare MurderTheHypotenuse. See also CockFight.
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!!Examples:

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[[folder: Anime and Manga ]]

* Spike and Vicious from ''Anime/CowboyBebop'' had one of these over Julia, resulting in the disintegration of their friendship and a lifetime of hatred toward each other.
* [[AnthropomorphicPersonification Turkey and Greece]] from ''AxisPowersHetalia'' were ''already'' on bad terms long beforehand, but them both liking Japan does not help one bit. According to France in the Christmas 2007 strips, it's bad enough that Japan spends most of his time with the two trying (vainly) to get them to be at least civil to each other.
* Brutal example in the ''VisualNovel/SchoolDays'' anime. Makoto starts out interested in Kotonoha, gets help from Sekai, and later falls for her, then goes back to Kotonoha at the end after he suspects everyone ditched him due to his lecherous ways by sleeping around with all the girls in the series, and Sekai's declaration that she's pregnant. [[spoiler: He gets knifed by a jealous [[{{Yandere}} Sekai]], who in turns gets killed by Kotonoha at the end.]]

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[[folder: Film ]]

* ''Film/ThePrestige''.
** Though the vendetta was pretty firmly in place before this came about.
* Played brilliantly in ''Film/SpiderMan'' where Mary-Jane is constantly the center of the tug-of-war between Spider-Man and the Goblin, Jr. AND their alter-egos Peter Parker and Harry Osborn.
* In the second ''[[Film/TheMatrix Matrix]]'' film Niobe used to be with the mystical Morpheus, now she is with the secular Locke. How much of the tension between the two men is a result of this and how much is due to their very different personalities and leadership styles is unclear since it all happened in the backstory.
** That Niobe keeps supporting Morpheus over Locke doesn't help matters much.

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[[folder: Literature ]]

* ''Literature/HarryPotter'': [[spoiler:Harry's mother used to be close friends with Severus Snape when they were children; he had an obvious crush on her as a teenager (althoughtv that no one but Dumbledore seemed to know), and a deep and abiding hatred for his then-rival, James Potter. Since Harry reminds Snape of his father, you've got some serious hostility there.]]
** Also completely played straight since [[spoiler: Lily initially didn't like James for being arrogant. However, her freindshipe with Severus was gradually breaking apart before it finally broke when Severus [[KickTheDog whipped out the]] FantasticRacism [[KickTheDog card on her]] after she tried to interfere with him and James. She ended up dating [[CharacterDepth the now less hot-headed ]] James, whom she eventually married and had Harry with.]]
* Creator/AlexandreDumas' ''Literature/TheCountOfMonteCristo''. This is particularly pronounced in the latest FilmOfTheBook, in which Mercedes is one of the key reasons behind [[spoiler:Fernand's plot to put Edmond in prison]].

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[[folder: Live-Action TV ]]

* ''{{Greek}}''. Cappie's carelessness and jealousy leads him to beat up his best friend and roommate Evan for taking Casey to a formal when he failed to show. This leads to the pairing of Casey and Evan at the beginning of the series and the bitter rivalry between Evan and Cappie's respective fraternities ever since.
* ''{{Smallville}}'': Lex and Lana are an item for a time, resulting in upped tensions between Clark and Lex.
* There is, of course, a ''Series/BuffyTheVampireSlayer'' example. Tension over Drusilla's affections is the main source of the feud between Angel and Spike for much of the second season, even leading to Spike helping Buffy at the end of the season in the hope he'll get Drusilla back.
** Ironically, Spike himself ends up romantically entangled with Angel's love interest, Buffy, in later seasons. These guys have VERY similar [[IncrediblyLamePun tastes]] in women.

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[[folder: Videogames ]]

* In the ''VisualNovel/PhoenixWrightAceAttorney'' series, one of the cases involves a couple of dueling celebrities (one which is a JerkAss and the other which is a CompleteMonster) and when the current fiancÚ (the JerkAss) found out that he had essentially gotten his rival's (the CompleteMonster's) leftovers, he immediately called off their engagement. The CompleteMonster had told him this ''just because he knew it would make his rival angry'' and this only escalated the rivalry between the two of them. The woman in the meantime? Tired of being TheChewToy, [[TearJerker she kills herself]].

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[[folder: Webcomics ]]

* Missi and Emily in ''Webcomic/{{Misfile}}'' to the point where Ash tells both of them to bug off until they can be decent to each other.

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[[folder: Web Original ]]

* ''DoctorHorriblesSingAlongBlog'': The opposites in this case are the VillainProtagonist Dr. Horrible and the HeroAntagonist Captain Hammer; the LoveInterest is WideEyedIdealist Penny, and Dr. Horrible attempts to MurderTheHypotenuse to claim his love (while simultaneously proving he's a real villain.) [[spoiler: She dies instead, leading to Dr. Horrible [[PyrrhicVictory being inducted into the Evil League of Evil]], his [[BeCarefulWhatYouWishFor major goal]] throughout the story.]]

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[[folder: Western Animation ]]

* Happens in ''TotalDramaWorldTour'' with Duncan, Courtney and Gwen.
* [=LaBarbara=] Conrad from ''WesternAnimation/{{Futurama}}'' has been married to both Hermes Conrad and his limboing opponent, Barbados Slim. In this case, they are opposites mostly in terms of Conrad being a short, chubby bureaucrat and Slim being an "ebony adonis" professional athlete.
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