A rulebook is a written collection of the rules for a game, sport, or organization. When you open a new board game and find that booklet explaining how to play, that's the rulebook. Baseball has an official rulebook that umpires consult to settle disputes about whether a play was legal. Schools have rulebooks (often called student handbooks) that explain what students can and cannot do.

Rulebooks exist because complex activities need clear, written guidelines that everyone can reference. Without one, people would constantly argue about what's allowed. When basketball players disagree about whether someone committed a foul, they can check the rulebook for the official answer. When two friends can't remember a specific rule in Monopoly, they look it up.

People sometimes say someone “goes by the rulebook” when they follow rules very strictly, or “throws out the rulebook” when they ignore established rules entirely. A referee who makes decisions by the rulebook sticks closely to the official regulations, even when it might be easier to be flexible.