The Arcane Ledger
Five lines that decide whether your magic is a system or a shortcut. A magic system stops being interesting the moment it stops refusing anything — when every problem has a spell, the story has no floor to stand on and the reader stops counting. This sheet makes you write the refusals down: what magic is, what it takes, what it cannot do, who is allowed near it, and who stops the ones who are not. One sheet per system, kept with the world.
Source — what it actually is
a thing, not a moodName the substance, not the feeling. “The weave” is fine if you can say what the weave is made of and where it sits when nobody is using it. If the honest answer is “everywhere, in everything”, write that down and treat it as the problem it is: a source with no address cannot be defended, hoarded, lost or run out of.
Cost — what every use takes
and who pays when the caster cannotA price nobody pays is not a price. Say what is spent, whether it comes back, and what happens when the person casting has run out — because that third answer is where most of your stories actually live. Costs paid only by the caster produce heroes; costs paid by someone else produce moral questions.
Hard limits — what it cannot do
write at least three, as rulesState the refusals in the same voice you would state the powers, because a limit phrased as a vibe is not a limit. “Magic cannot truly raise the dead” is usable at a table; “resurrection is difficult” is an invitation to argue. Three is the working minimum — with fewer, one clever caster solves your whole plot and you have to invent a reason they cannot.
Practitioners — who can do it
made or born, and how rareSay who can, how they come to be that way, and roughly how many exist in the world you are showing. Then say how a stranger can tell — a mark, a licence, a habit, the way they hold their hands. Rarity you have not counted will drift the moment you need a fourth practitioner for a scene.
Enforcement — who stops the misuse
and what they do about itEvery limit needs a party with both the authority and the means to enforce it, or the limit is decoration. Name who holds that authority, where it comes from, and its failure mode — a guild that can be bought, a crown that can be outrun, a god that has stopped answering. The failure mode is not a flaw in the system; it is the door your antagonist walks through.
A system is what it refuses
Magic with no ledger is not a system, it is a mood — and moods cannot be argued with. The five lines here are not a power list. They are the answer sheet to the questions a reader or a player will ask the first time magic does something convenient: where did that come from, what did it cost, why has nobody done it before, and who was supposed to stop them.
That last one is the line most systems skip, and it is the one that pays best. Source and cost explain the world; enforcement explains the story, because somebody has to be failing to enforce something for there to be a plot at all. If your answer is “nobody enforces it”, write that down deliberately and make the anarchy load-bearing rather than leaving it as an accident.
Revise it in place as the world grows, so there is only ever one ledger and it is true now. When you add a power later, add it against these five lines first — if it fits inside them, it is the same system; if it does not, you have either found a limit worth breaking on purpose or a problem worth admitting.