The rules and constraints of a game define what can or cant be done in a game. They lay down the framework, or model, within which the game shall take place. Rules regulate the development of the game and determine the basic interactions that can take place within it. (Zagal, Nussbaum et al. 2000) A games rules should not be considered static or fixed. In fact, some games, like Fluxx, are based on the idea that the games rules are constantly changing as the game is played. (Looney 1998)
Looney, A. (1998). Fluxx, Looney Laboratories, Inc.
Zagal, J., M. Nussbaum, et al. (2000). "A Model to Support the Design of Multiplayer Games." Presence: Teleoperators and Virtual Environments 9(5): 448-462.