How games are built on the foundations of rules, and how rules—of which there are only five kinds—really work. Board games to sports, digital games to party games, gambling to role-playing games. They all share one thing in common: rules. Indeed, rules are the one and only thing game scholars agree is central to games. But what, in fact, are rules? In The Rule Book, Jaakko Stenros and Markus Montola explore how different kinds of rules work as building blocks of games. Rules are constraints placed on us while we play, carving a limited possibility space for us. They also inject meaning into our play: without rules there is no queen in chess, no ball in Pong, and no hole in one in golf. Stenros and Montola discuss how rules constitute games through five foundational types: the explicit statements listed in the official rules, the private limitations and goals players place on themselves, the social and cultural norms that guide gameplay, the external regulation the surrounding society places on playing, and the material embodiments of rules. Depending on the game, rules can be formal, internal, social, external, or material. By considering the similarities and differences of wildly different games and rules within a shared theoretical framework, The Rule Book renders all games more legible.