So far we have discussed rules in a relatively straightforward way. For example, we looked at the rules of Tic-Tac-Toe as they might appear in an instructions manual and argued that these rules fully constitute the rules of the game, the complete formal structure of Tic-Tac-Toe. Here are those rules one more time:
Play occurs on a 3 by 3 grid of 9 empty squares.
Two players alternate marking empty squares, the first player marking Xs and the second player marking Os.
If one player places three of the same marks in a row, that player wins.
If the spaces are all filled and there is no winner, the game ends in a draw.
Do these four rules constitute the complete formal system of Tic-Tac-Toe? Although these rules do describe to players what they need to know in order to play, there are aspects of the formal system of Tic-Tac-Toe that are not included here. Specifically, there are two kinds of formal structures that these four rules do not completely cover: the underlying mathematical structures of the game and the implied rules of game etiquette.