By Jackson Dix
There are two halves to worldbuilding. One is texture: food, weather, slang, what people do on holidays. The other is rules: what is possible in this world, what it costs, and who controls it. Texture makes a setting feel lived in. Rules make the plot work.
This post is about the rules half. If you’re working out how to build a fantasy world, or a science fiction one, it’s the half to settle first. A rule is anything in your setting that changes what a character can do. A spell, a faster-than-light drive, a law about who can own land, a curse that kills anyone who lies under a full moon. If it changes a character’s options, it’s a rule, and readers will test it.
Readers test rules the way players test a game. They notice when a hero could have solved chapter four’s problem with the trick from chapter nine. They notice when a technology that ended a war in the prologue is never mentioned again. You don’t need a fifty-page appendix to avoid this. You need to ask each rule a few hard questions before you lean on it.
Below are five questions I run on every major rule in a setting. For each one there’s an explanation, an original illustration, and a quick fix if the rule fails. The illustrations come from two settings invented for this post: a fantasy city where spoken promises are binding magic, and a far-future colony that runs on borrowed time.
Question 1: What does it cost?
A rule with no cost becomes the answer to every problem. If teleportation is free, why does anyone ride a horse? If the healer can fix any wound, why is anyone afraid of a sword?
Cost doesn’t have to mean money. It can be time, pain, memory, years of life, social standing, or a debt to someone dangerous. The best costs hurt the character who pays them in a way that matters to the story.
Illustration: the debt city
In the city of Harrowgate, any spoken promise witnessed by two people becomes binding. Break it and you lose the use of your hands until you make it right. The cost of using this power is that you can only bind others by binding yourself. Every contract has two sides. A rich merchant who holds a hundred promises also owes a hundred.
The fix if it fails
If your rule costs nothing, attach a cost that grows with use. Small uses are cheap. Big uses are expensive. That alone keeps the rule from flattening your climax.
Question 2: What are its limits?
Cost answers “what do you pay?” Limits answer “what can’t it do, at any price?”
Limits are where your plot lives. A hero can’t solve a problem with magic if the problem sits outside magic’s reach. Ursula K. Le Guin’s Earthsea books are a well-known case. Magic there works through true names, so a wizard’s power ends where his knowledge of names ends. That single limit shapes what wizards study, what they fear, and what they can’t fix.
Illustration: the borrowed-time colony
On the colony world Tenley, a device called a lender can slow time inside a small field. A medic can stretch one minute into an hour to perform surgery. The limits: the field can’t be larger than a room, it can’t run for more than six hours of outside time, and nothing living can cross its boundary while it runs. That last limit is the one the plot uses. A character trapped inside a field when the colony is attacked can’t get out to help.
The fix if it fails
Write down three things the rule can never do. Then check that at least one of them gets in your protagonist’s way at a moment that hurts.
Question 3: Who controls it?
Every rule creates winners and losers. Ask who has access, who is shut out, and who profits from keeping it that way.
This question turns a magic system or technology into politics, and politics gives you conflict without needing a villain to invent it. Frank Herbert’s Dune is a famous example. The spice is needed for space navigation and comes from one planet, so every faction in the galaxy has a reason to fight over that planet. The rule creates the empire’s politics.
Illustration: the debt city
In Harrowgate, the Witness Guild licenses who may serve as a witness to a binding promise. Without two guild witnesses, a promise is just words. So the guild controls every contract in the city. Unlicensed witnessing is a crime. A street kid who discovers that her testimony binds just as well as a guild member’s has found something the guild will kill to hide.
The fix if it fails
If anyone can use your rule equally, ask what would happen in a real society. Someone would try to control it. Someone would try to tax it. Write down who that is.
Question 4: What has it changed?
A rule that has existed for centuries would reshape the world around it. Architecture, law, religion, crime, warfare, and daily habits would all adapt.
This is where many settings break. The author adds a powerful rule and then writes a society that looks like it never had it. If healing magic is common, life expectancy changes, medicine changes, and the way wars are fought changes. If people can read minds, privacy works differently and so does a courtroom.
You don’t have to show every change. Pick a few and show them in passing. Readers fill in the rest.
Illustration: the borrowed-time colony
On Tenley, schools use lenders for exams so students can take a three-hour test in thirty outside minutes. Prisons are lenders run in reverse: one outside day, a week inside. Lovers on night shifts rent small fields to stretch an evening. None of this is plot. It’s a few lines in different scenes that make the device feel old.
The fix if it fails
For each major rule, write one change in each of these areas: work, law, and daily life. Use them as background details. Add more areas if the rule is big enough to need them.
Question 5: Where does it break?
Rules that never break can’t surprise anyone. The most interesting moments in a speculative plot often come when a rule does something its users didn’t expect.
This isn’t the same as cheating. A rule that breaks in a way the author set up earlier feels earned. A rule that breaks only when the plot needs a rescue feels like a cheat. Isaac Asimov built many robot stories around this. The Three Laws of Robotics were clear, and the stories came from situations where the laws produced strange results nobody planned for.
Illustration: the debt city
Harrowgate’s promise magic binds anyone who speaks a promise in front of two witnesses. Nobody has thought about what happens when the promise is made to the city itself. The founder of Harrowgate did exactly that three hundred years ago, and the promise was never fulfilled. That’s the secret your protagonist uncovers in act three, and it works because the rule was stated plainly in chapter one.
The fix if it fails
Find the edge case. What happens when the rule meets itself, meets another rule, or meets a situation its creators never imagined? Plant that edge case early, then use it later.
Writing the rules into the story
Once your rules pass these questions, you still have to get them onto the page without stopping the story for a lecture. A few habits help.
Show the rule before you need it
If a rule will save or doom someone in the climax, show it working in a low-stakes scene first. The reader learns how it works while nothing is on the line. When it matters, they already understand.
Let characters disagree about it
People in your world won’t all understand the rules the same way. A priest, a scientist, and a smuggler might explain the same magic three different ways. That conflict teaches the reader and builds character at once.
Keep a rules file
Write each rule, its cost, its limits, who controls it, what it changed, and where it breaks. One page per rule. Check it when you draft scenes that use the rule. This is the least glamorous part of building a setting and the one that saves the most revision time.
Tie at least one rule to your antagonist
The strongest SFF antagonists understand a rule better than anyone or exploit a gap in it. If your villain could be dropped into a story with no speculative elements, you’re not using your setting. See Writing Villains Who Want Something: An SFF Build Sheet for a full method on building a villain around a world rule.
How rules connect to plot and character
Rules don’t sit apart from the rest of your story. A cost should force a character to choose. A limit should block a plan. A break should change what the reader thought was possible.
When you outline, look for places where a rule can create pressure. The plot tends to come out of the gap between what a character wants and what the rules allow. For how to build a plot around that gap, see Plotting SFF Novels: Plot Backward From the Climax. For how a rule can shape what a character believes and how that belief changes, see Character Arcs in SFF: Map the Belief That Has to Change.
FAQ
How many rules should a magic system have?
Enough to cover what the story uses. A magic system needs clear costs and limits for every power that affects the plot. Powers that only add texture can stay vague. If a power solves or causes a major problem, define it.
Do science fiction settings need rules as strict as fantasy magic?
Yes. A faster-than-light drive, a mind upload, or an AI is a rule in the same sense a spell is. Readers will check its costs, limits, and effects on society just as hard. The vocabulary changes, but the questions are the same.
How do I explain my world’s rules without an info dump?
Show each rule working in a low-stakes scene before the story depends on it. Let characters argue about how it works. Give the reader only what they need for the next scene, and trust them to collect the rest.
What if I break my own rule by accident?
Fix it in revision by either changing the scene or changing the rule. If the break makes the story better, plant the edge case earlier so it reads as a discovery instead of a mistake. A rules file makes these breaks easier to catch.