Rules are the constraints that define what actions are permitted, prohibited, or required within a game. They create the space of meaningful choice by limiting what players can do, which is what makes a game a game rather than free play.
Rules need not be written. Children’s games often operate on negotiated consensus; folk games transmit rules orally across generations; and many social games rely on implicit norms that only become visible when someone violates them. What matters is not the format but the function: rules set up shared expectations about how the game works.
A useful distinction is between constitutive rules (which define what the game is — chess without the movement rules is not chess) and regulatory rules (which govern conduct — tournament etiquette, time limits, sportsmanship norms). Constitutive rules determine the mechanics; regulatory rules determine the social context. House rules sit in between, modifying constitutive rules by local agreement.