Unwritten describes rules, agreements, or understandings that everyone follows even though no one has officially written them down. In every classroom, there are unwritten rules about behavior: you probably know not to interrupt when someone's sharing something important, even if that's not posted on the wall. At the lunch table, your group might have unwritten rules about where each person sits, without ever discussing it formally.

These invisible guidelines shape how groups work together. A sports team might have an unwritten rule that veterans get first choice of jersey numbers. Families develop unwritten rules about holidays, mealtimes, and helping with chores. Sometimes unwritten rules are more powerful than written ones because people follow them out of respect and tradition rather than because someone's enforcing them.

The word can also mean something that hasn't been written yet. A blank page holds an unwritten story waiting for its author. An unwritten chapter of history refers to events that haven't been recorded or that historians don't know much about yet.

When someone says “that's an unwritten rule,” they're pointing out something important that newcomers need to learn by watching and listening, not by reading a handbook.