to play, they choose the first available move from the following list, as used in Newell and Simon's 1972 tic-tac-toe program. Win: If the player has two in a row, they can place a third to get three in a row. Block: If the opponent has two in a row, the player must play the third themselves to block the opponent. Fork: Cause a scenario where the player has two ways to win (two non-blocked lines of 2). Blocking an opponent's fork: If there is only one possible fork for the opponent, the player should block it. Otherwise, the player should block all forks in any way that simultaneously allows them to make two in a row. Otherwise, the player should make a two in a row to force the opponent into defending, as long as it does not result in them producing a fork. For example, if "X" has two opposite corners and "O" has the center, "O" must not play a corner move to win. (Playing a corner move in this scenario produces a fork for "X" to win.) Center: A player marks the center. (If it is the first move of the game, playing a corner move gives the second player more opportunities to make a mistake and may therefore be the better choice; however, it makes no difference between perfect players.) Opposite corner: If the opponent is in the corner, the player plays the opposite corner. Empty corner: The player plays in a corner square. Empty side: The player plays in a middle square on any of the four sides. The first player, who shall be designated "X", has three possible strategically distinct positions to mark during the first turn. Superficially, it might seem that there are nine possible positions, corresponding to the nine squares in the grid. However, by rotating the board, we will find that, in the first turn, every corner mark is strategically equivalent to every other corner mark. The same is true of every edge (side middle) mark. From a strategic point of view, there are therefore only three possible first marks: corner, edge, or center. Player X can win or force a draw from any of these starting marks; however, playing the corner gives the opponent the smallest choice of squares which must be played to avoid losing. This might suggest that the corner is the best opening move for X,