When you write secondary world fiction (anything set on a world you made up), people expect to it to be separated from Earth. To do this, you need to maintain a perspective on just what is specific to our world, and what are just things that any human culture might have come up with. Much of it comes down to slang and idioms, but some concepts tie back to specific historical or religious touchstones peculiar to Earth. Not everyone will notice these slips, but when you make them, you can break the immersion of a reader who does.
When you make up a world (or worlds), you have taken yourself away from the physical geography of Earth. Any references you to Earth’s geographic features will ring false. You can’t compare a mountain to Mt. Everest. There will be no puns about denial being a river in Egypt. Nothing may be dry as the Sahara or clear as Caribbean waters.
Scour your prose and dialogue alike for anything that requires knowledge of Earth geography to convey its meaning. If you must keep the idiom, alter it to suit your world. Substitute your own tallest mountain or widest desert. Better yet, craft new idioms that don’t require the references.
A lot of historical figures have things named for them. These don’t have places in your world, because those people should never have existed. No one in your fictional world should have a Napoleon complex, be called a Benedict Arnold, or sign their John Hancock on a document. You can cut a huge swath out just be eliminating things named after Caesar (from the month of July, to a variety of chicken salad, to the procedure for surgically assisted birth, as well as a variety of titles for rulers across Europe).
Exception: I will forgive the existence of sandwiches. They might be named after the Earl of Sandwich, but I can’t think of another good name for them, few people closely associate the word with the good Earl, and I would have to imagine that any society would have to develop them eventually, if they were to survive.
This can be a tough one to root out, since so much of our world is infused with religious context. It can be hard to realize just how much is specific to Earth. Here’s a quick rundown of things that you might want to be wary of.
- Days of the week
- References to hell, heaven, damnation, zen, nirvana, pope, jihad
- Blessing someone who sneezed and other old superstitions
You can reincorporate any or all of these with some modification to make them apply to your own world, but give some thought to the origin of the terminology you use before allowing it into your story.
This is probably one of the easier ones to spot. Don’t let Earth’s pop culture pollute your world. This can be as subtle as old Shakespearean references (which was 16th C. pop culture), or slipping in Beatles lyrics, or as blatant as having your characters quote Star Wars or The Princess Bride. Those might have been some great quotable sources, but your characters should never have heard of them.
Related to this is popular slang. Cool, groovy, far out, awesome, radical, wicked, tight, slick, or epic; whatever you’ve got stuck in your head from your days on Earth, leave it at the side of your desk while you’re writing.
A lot of fiction has wormed its way into the common lexicon. That’s fine for us Earth-dwellers, but as a writer you’ve got to watch out for references that should never have had a chance to make it into your world’s dictionaries. The best example of this I can think of is the Achilles heel/tendon. Having an Achilles heel is a metaphoric reference that you need to understand the story of Achilles to process. Without that background, you’d have a Darmok moment. The Achilles tendon is a trickier subject, because that’s the actual name of a part of human anatomy. I would rework the Achilles heel into a metaphor appropriate to your own world, while I would try to dance around tendon injuries to the leg.
This warning also applies to Trojan Horse viruses, genies in bottles (unless your world shares a similar method for genie storage), and fairy godmothers.
Generally, I think most writers can keep a company name out of their writing without too much help. But particularly in modern or futuristic settings, you have to be wary of words that have become part of the common lexicon, despite being proprietary company names or brands from here on Earth. Xerox, Kleenex, Coke, Band-Aid, and a whole host of others have become common terms that wouldn’t have come into being in other worlds.
Special note: Alcoholic beverages are very commonly branded, often involving the name of the company or region of their origin. Study up on which are the generic names and which the brands. Whisky is generic; Scotch requires a Scotland to exist. Also watch our for Champagne, Guinness, and Tequila.
Sports idioms are pervasive in our language. Punt, home run, strikeout, buzzer beater, pinch hitter, and quarterback are all common phrases whose usage have been stretched beyond their sporting origins (though the meanings are generally the same). Consider those to be off limits to worlds that have never heard of baseball, football, basketball, and the like. Instead, consider what sports of leisure games your characters would use instead as their cultural touchstones.
Bonus – Flipping the script
What if your world is Earth, or descended from Earth (lost colony, post apocalyptic)? You can intentionally break these rules to give your reader hints. It might be jarring to your reader at first, but a properly laid trail of clues should build toward your grand reveal.