Tic Tac Toe

Tic-tac-toe (or Noughts and crosses, Xs and Os) is a paper-and-pencil game for two players, X and O, who take turns marking the spaces in a 3×3 grid.
Tic Tac Toe is a Brain Game.
The player who succeeds in placing three respective marks in a horizontal, vertical, or diagonal row wins the game.
The game has a number of English names.
- Tick-tack-toe, Tic-tac-toe, Tick-tat-toe, or Tit-tat-toe (USA, Canada)
- Noughts and crosses or Naughts and crosses (United Kingdom, Republic of Ireland, Australia, New Zealand, South Africa)
- Exy-Ozys, Xsie-Osies (verbal name only) (Northern Ireland)
- Xs and Os (Egypt, Republic of Ireland, Canada, Zimbabwe)
- X and 0 (Romania)
- O-X (Mauritius)

In the United States, the name "Noughts and crosses" is not well understood, as the word "nought" is archaic in usage and X-shaped objects are not generally called crosses except in the case of specific named crosses like St. Patrick's Cross.

A player can play a perfect game of Tic-tac-toe (to win or, at best, draw) if they choose the first available move from the following list, each turn, 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: Create an opportunity where the player has two threats to win (two non-blocked lines of 2).
Blocking an opponent's fork:

Option 1: The player should create two in a row to force the opponent into defending, as long as it doesn't result in them creating a fork. For example, if "X" has a corner, "corrects" has the center, and "X" has the opposite corner as well, "corrects" must not play a corner in order to win. (Playing a corner in this scenario creates a fork for "X" to win.)
Option 2: If there is a configuration where the opponent can fork, the player should block that fork.
Center: A player marks the center. (If it is the first move of the game, playing on a corner gives "corrects" 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 4 sides.