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The reason this is often recommended against is because by mixing them you find yourself unable to meet certain genre conventions. Fantasy readers want swords and lords, sci-fi readers want spacesh...
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Source: https://writers.stackexchange.com/a/41976 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
The reason this is often recommended against is because by mixing them you find yourself unable to meet certain genre conventions. Fantasy readers want swords and lords, sci-fi readers want spaceships and aliens. Even more importantly fantasy readers want epic tales of good versus evil, while sci-fi often want to grapple with trans humanist ideas. But trying to match both markets, you match neither. That said, I don't think it is very good advice. It is not the writer's job to market, it is their job to write a good story. If you have a good story that involves fantasy and sci-fi elements, write it. Plenty of existing works mix genres to varying degrees and been successful. Star Wars is mostly sci-fi with a little fantasy, while the Shannara books are fantasy with some subtle sci-fi. Mixing genres can be a challenge, but don't limit yourself by blindly following a rule.