Rules constitute the inner, formal structure of games. All games have rules, and rules are one of the defining qualities of games.
Rules are not the experience of play. It is possible to make experiential changes to a game (such as changing the names of the four suits in a deck of cards) without changing the rules or formal structures of a game.
Game rules as considered under the Primary Schema of RULES are different than rules of strategy. Strategic "rules of thumb" help players to play a game, but do not define the formal identity of a game.
Game rules are different than the rules of etiquette, law, war, or other social rules. Games are intrinsically artificial and separate from "real-world" contexts, whereas these other forms of rules are not separate from ordinary life.
Following are the general characteristics that all game rules share:
Rules limit player action
Rules are explicit and unambiguous
Rules are shared by all players
Rules are fixed
Rules are binding
Rules are repeatable
Although some games question and violate these characteristics, these are the common traits of game rules considered from a strictly formal point of view.