tictactoe

Tic-tac-toe, also known as noughts and crosses, is a simple two-player pencil-and-paper game played on a 3x3 grid. Players alternately place marks, typically X first and O second. The objective is to form a straight line of three marks horizontally, vertically, or diagonally.

Rules: The game starts with an empty grid. Players take turns placing their mark in any empty cell. If a player completes a line of three, that player wins immediately. If all nine cells are filled without a winner, the game ends in a draw.

Strategy and outcome: With perfect play, tic-tac-toe ends in a draw. The center square is considered the strongest opening move because it creates the most opportunities to form winning lines, with corners typically offering better chances than edge positions. Players may create tactical forks—positions that give two possible winning lines—forcing the opponent to defend and often leading to a draw.

Computation and pedagogy: The game is solved; the optimal moves and outcomes are known. It is widely used in computer science education to illustrate search algorithms such as minimax, as well as concepts of recursion, decision trees, and game-tree complexity in a simple setting.

Variants and cultural notes: The game is known as tic-tac-toe or noughts and crosses in different regions. Variants exist on larger boards, including four-by-four or five-by-five grids and three-dimensional versions, as well as alternative win conditions or misère rules. Despite its simplicity, tic-tac-toe remains a common teaching tool and casual pastime.