Page "Game tree" Paragraph 2

The diagram shows the first two levels, or plies, in the game tree for tic-tac-toe.

We consider all the rotations and reflections of positions as being equivalent, so the first player has three choices of move: in the center, at the edge, or in the corner.

The second player has two choices for the reply if the first player played in the center, otherwise five choices.

And so on.