If finite games must be externally bounded by time, space, and number, they must also have internal limitations on what the players can do to and with each other. To agree on internal limitations is to establish rules of play.

The rules will be different for each finite game. It is, in fact, by knowing what the rules are that we know what the games is.

What the rules establish is a range of limitations on the players: each player must, for example, start behind the white line, or have all debts paid by the end of the month, charge patients no more than they can reasonably afford, or drive in the right lane.

In the narrowest sense, rules are not laws; they do not mandate specific behavior, but only restrain the freedom of the players, allowing considerable room for choice within those restraints.

If these restraints are not observed, the outcome of the game is directly threatened. The rules of a finite game are the contractual terms by which the players can agree who has won.