A magic system is not a list of spells. It is a stack of decisions, and the components are the decisions.
Every working system in fantasy answers the same handful of questions before the plot ever starts moving. Where does the power come from. Who gets to use it. What does it take from them. What can it not do. How is it actually cast. What does it do to the world.
Miss one of those and the reader can feel the gap even if they cannot name it. A component you have not decided is a hole your reader falls through, usually in the third act, right when you needed them to believe the stakes.
So this is the anatomy. Each component below is one decision. For each I will say what it is, what it decides for your story, and what goes wrong when you leave it vague.
Almost every one comes down to the same three things underneath: cost, access, and consequence.
Here are the seven components, in the order this guide works through them:
- Source. Where the power comes from
- Access. Who gets to use it
- Cost. What it takes from the user
- Limits and rules. What it cannot do
- Mechanism. How it is actually cast
- Consequences. What it does to the people using it
- Cost to the world. What it does to everyone else
Source: where the power comes from
The source is the well. It is the answer to "why does magic exist here at all," and it quietly decides who can reach the well and who can be cut off from it.
Sanderson's Allomancy is the clean example. The power comes from ingesting and burning specific metals, so the source is a physical, ownable, exhaustible thing. That single decision hands you an economy, a smuggling trade, and a way to disarm a magic user by taking their vials.
Compare that to power that comes from "the ancient energy of the world." Fine. But who controls it, who is born near it, who can be starved of it? If the source cannot be owned or withheld, it cannot create conflict.
The failure mode is a source that is everywhere and belongs to no one. That is not a magic system, that is weather. You can build the rest of your system on top of a shaky source, but see how the origins of a magic system tend to decide half of everything downstream.
Access: who is allowed to use it
Source decides where power lives. Access decides who gets to touch it. This is where your social structure comes from, whether you meant to build one or not.
There are only a few real answers. Magic is innate, meaning you are born with it. It is learned, meaning anyone with time and a teacher can get it. It is granted by something outside you. Or it is bought.
Each one hands you a different world. Innate access gives you a caste, a bloodline, a class of people who are simply born above or below everyone else. Learned access gives you schools, apprenticeships, gatekeepers who decide who gets trained.
Harry Potter runs on born access, which is why the whole society splits neatly into wizards and Muggles with almost no one in between.
Never decide this and magic quietly becomes available to whoever the plot needs it to be available to, until the reader stops trusting that anyone is actually limited. If you want to see how far this single decision reaches, the way magic shapes who holds power is usually the most political part of a fantasy world.
Cost: what it takes from the user
This is the one thin systems skip, and it is the most important component you have.
Cost is what using magic takes out of the person using it. Energy, blood, memory, time, sanity, money, years off their life. Without a real cost, magic is just a button the hero presses to win, and a button that always works is boring by the second time they press it.
Rothfuss builds sympathy in The Name of the Wind almost entirely out of cost. Moving energy from one place to another loses heat, and if you have nothing to draw from, you pull that heat out of your own body and freeze yourself doing it. You believe the magic because you can see what it costs.
Watch out for magic that only costs "concentration" or "being tired." That is a cost the reader cannot picture and cannot fear. When you do not know the cost, look hard at how your system handles rules and limits, because a vague cost usually means the whole system is soft where you needed it hard.
Limits and rules: what it cannot do
Cost is what magic takes. Limits are what it simply cannot do, no matter what you pay.
Limits are how the reader learns to predict outcomes, and prediction is where tension comes from. If the reader cannot guess whether the hero can escape the locked room with magic, they cannot feel the danger of the locked room.
Sanderson's law says it plainly enough that people quote it: your ability to solve problems with magic is proportional to how well the reader understands its limits.
Named limits also stop the deus ex machina before you are tempted to write one. If your magic explicitly cannot bring back the dead, the reader trusts the death.
The failure mode is a system that can apparently do anything, revealed one convenient new ability at a time. The distinction between a hard magic system and a soft magic system is really just how many of these limits you have chosen to make explicit to the reader.
Mechanism: how it is actually cast
The mechanism is the physical act. Words, gestures, materials, focus, a wand, a drawn circle, a true name spoken aloud. It is the texture of your magic, and it decides how magic can be blocked, stolen, or faked.
Le Guin's Earthsea runs on true names. To work magic on a thing you must know its real name, which means magic is an act of knowing, and the mechanism is fundamentally about language and restraint. Harry Potter runs on wand plus incantation, which is why disarming a wizard means taking their wand, a plot move the mechanism made available for free.
The mechanism is also where your world gets its feel. A system cast through muttered Latin and a stick feels nothing like one cast through blood on a stone floor.
If a reader ever asks how exactly a spell got cast and you cannot answer, the mechanism is too vague and magic just "happens" when characters want it to. Decide the physical act and half your magic system's texture and detail shows up on its own.
Consequences: what using it does to the world
Consequences are where the limits earn their keep in the plot.
A rule the reader never sees enforced is a rule the reader does not believe. Someone has to break the taboo, pay the cost, and show the rest of the world watching.
Earthsea's whole moral spine is this. Every act of magic risks tipping the Equilibrium, so the consequence of casting is not personal, it is cosmic, and the wisest mages are the ones who mostly choose not to cast.
Consequences are frequently the most human part of a fantasy world, because everyone has lived under a system that gives just enough while quietly taking more.
The failure mode is magic with no wake behind it. Spells get cast, problems get solved, and the world resets to neutral by the next chapter. Nothing that powerful would leave the world unchanged.
Cost to the world: who profits, who pays, who regulates it
The last component is the one most writers never reach, and it is where source, access, and cost all come home to roost.
If magic is powerful and scarce, someone owns it. Someone profits from it, someone is priced out of it, and someone shows up to regulate it. Allomancy's dependence on metals gives Sanderson a ruling class built on controlling that supply.
The moment you decide magic has a real cost and limited access, you have accidentally decided who is rich and who is exploited.
This is not a bonus round. It is the difference between a magic system and a magic economy, and readers feel the second one as real in a way the first never manages.
Skip this and you get a world where magic is world-shaking on the page and somehow changes nobody's daily life around it. If you want your system to feel lived in rather than performed, this is the component to sit with. Plenty of the most unique magic systems got that way by taking this one seriously.
The components at a glance
| Component | The decision it makes | Failure mode if skipped |
|---|---|---|
| Source | Where power comes from, and who can be cut off | A source no one owns creates no conflict |
| Access | Who can use it, and the class structure that follows | Magic quietly appears wherever the plot needs it |
| Cost | What it takes from the user | A button that always works, boring by the second press |
| Limits and rules | What it cannot do, so the reader can predict | Anything is possible, revealed one convenient power at a time |
| Mechanism | How it is physically cast, blocked, or stolen | Magic just "happens" when characters want it |
| Consequences | What it does to the world after casting | Spells solve problems, world resets to neutral |
| Cost to the world | Who profits, who pays, who regulates it | World-shaking magic that changes nobody's daily life |
Run your own system through the components
Take your system and go down the list. Source, access, cost, limits, mechanism, consequences, cost to the world.
Here is one of mine with all seven answered. In The 13th House, the source is the sign you were born under. Access is total and involuntary, everyone gets exactly one. The cost is your own lifespan, since using your sign spends years you do not get back. The limit is that your power is fixed to your sign and walled off from the others. The mechanism is the sign itself, a Cancer throws an unbreakable wall around anyone they love and never around themselves. The consequence is a life measured in what you were willing to spend it on. The cost to the world is a whole society sorted by birth sign, which decides your worth before you have done anything.
Compare that to a world where the cost lands on everyone at once. In Mudwick, drawing power drains the emotional residue out of a place, so the cost-to-the-world component is an extraction economy, with the people who control the richest sites at the top and the drained at the bottom.
For each one, either you can say the answer in a sentence or you cannot. The ones you cannot answer are not gaps you will fill later. They are the exact places a reader will stop believing you, and you will not know which chapter it happened in.
Most thin magic systems are not thin everywhere. They are thin in one or two components, usually cost and cost-to-the-world, and the rest is dressed up well enough to hide it. Fixing a system almost always means finding the one vague component and making a real decision there.
If you are still assembling the raw material, a stack of magic system ideas or some inspiration to work from helps, and running through the various types of magic systems shows you which shapes have already decided these components for you.
FAQ
What are the components of a magic system? A magic system has seven components: source, access, cost, limits and rules, mechanism, consequences, and cost to the world. Each one is a decision: where power comes from, who can use it, what it takes from the user, what it cannot do, how it is cast, what it does to the world, and who profits or pays for it.
Which component matters most? Cost. It is the one thin systems skip, and without it magic becomes a button that always works, which kills tension. If you fix only one vague component, make it the cost your magic takes from the person using it.
Do all magic systems need every component? Every system has every component whether you decided it or not. A soft magic system leaves some of them deliberately unexplained to the reader while a hard one makes them explicit, but the undecided ones still shape the story, usually as plot holes.
How do I know which component I have left vague? Try to answer each one in a single sentence. Where does power come from, who can use it, what does it cost, what can it not do, how is it cast, what does it do to the world, who profits. The questions you cannot answer cleanly are your gaps.
What is a good example of a magic system with clear components? Sanderson's Allomancy. The source is ingesting specific metals, access is innate ability plus owning the metals, cost is burning through a finite supply, and the limits are fixed per metal. Every component is decided, which is why the whole system holds up under pressure.
