Rule 1 of 24 · Chapter I — The Laws Beneath the Story
Keep faith with your own rules
Why this rule exists
An invented world does not ask the reader to believe it is real; it asks them to behave, for a while, as if it were. That contract holds only as long as the world keeps its word. The moment a thing that was impossible in chapter three becomes trivial in chapter nine, without a reason the story has earned, the reader stops living inside the world and starts watching an author move furniture. Consistency is not pedantry or a love of spreadsheets. It is the single quality that separates a world you can stand inside from a set of pretty backdrops, because a reader can forgive almost any strangeness except the sense that the rules will change whenever the plot gets stuck.
In practice
Decide your world's rules early and treat them as law, not suggestion. When you need something new, do not bolt it on where it is convenient; ask what else that rule implies and whether it contradicts anything you have already shown. Keep a running list of every constraint you have established on the page, so that past you and future you are working for the same world. When a rule genuinely must change, change it on purpose and in view of the reader, so it reads as revelation rather than cheating. Reread with a suspicious eye, hunting for the small betrayals: the wound that heals too fast, the distance that shrinks when the hero is late.
When it doesn't apply
A revealed rule can deepen without breaking, so long as the deepening does not contradict what came before. And a story that is openly about instability, dream-logic, a reality coming apart, may make inconsistency its subject rather than its flaw. The test is whether the reader feels a promise kept in a new way, or simply broken.