# Tic-tac-toe (game)

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May 23, 2022

Tic-tac-toe (also known as row three, tria, cross and dot, zero per, fillet, circles and crosses, OXO, XOXO) is a hugely popular full information abstract pencil and paper game. It is played on a 3 × 3 square grid. In turn, the players choose an empty cell and draw their symbol on it (usually a player has an "X" as a symbol and the opponent a circle). The player who manages to arrange three of his symbols in a straight horizontal, vertical or diagonal line wins. If the grid is filled without either player completing a straight line of three symbols, the game ends in a draw. In the event that the game ends in a draw, the game is called a "draw", as in the game of chess. There are old finds in ancient Rome, where the soldiers played with each other; the game took the name of terni lapilli. Game is often used as a pedagogical element to explain game theory and the basics of artificial intelligence. It is in fact a game with perfect information with zero sum, of which the "perfect strategy" is known, that is the Nash equilibrium. Since the number of possible situations (the complexity of state space) is extremely small compared to other games, writing a computer program that plays tic-tac-toe perfectly, without ever losing, is a task within the reach of any programming student. The trio was in fact the first game in history to be played on a computer; the program was OXO, developed for the EDSAC computer (1952).