Tic-tac-toe

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“Noughts and Crosses” redirects here. For the Noughts and Crosses book and series, see Noughts & Crosses series.

Tic-tac-toe, also called noughts and crosses, hugs and kisses, and many other names, is a pen-and-paper game for two players, O and X, who take turns to mark the spaces in a 3×3 grid. The player who succeeds in placing three of their own marks in a horizontal, vertical or diagonal row wins the game.

This game is won by the first player, X:

This is a "cat's game", that is, a draw:

Players soon discover that best play leads to a draw, regardless of where the first player plays. So tic-tac-toe is most often played by very young children; when they have discovered an unbeatable strategy they move on to more sophisticated games such as dots and boxes. This reputation for ease has led to casinos offering gamblers the chance to play tic-tac-toe against trained chickens.^[1]^[2]

The first two plies of the game tree for tic-tac-toe.

The simplicity of tic-tac-toe makes it ideal as a pedagogical tool for teaching the concepts of Combinatorial game theory and the branch of artificial intelligence that deals with the searching of game trees. It is straightforward to write a computer program to play tic-tac-toe perfectly, to enumerate the 765 essentially different positions (the state space complexity), or the 31,896 possible games up to rotations and reflections (the game tree complexity) on this space.

Without eliminating symmetries, there are 255,168 possible games. Assuming that X makes the first move every time,

131,184 games are won by X,
77,904 games are won by O,
46,080 games are a draw.

The first known computer game, OXO (or Noughts and Crosses, 1952) for the EDSAC computer played perfect games of tic-tac-toe against a human opponent.

One example of a Tic-Tac-Toe playing computer is the Tinkertoy computer, developed by MIT students, and made out of Tinker Toys. It only plays Tic-Tac-Toe, and has never lost a game. It is currently on display at the Museum of Science, Boston.

1 Strategy
2 Variations
3 Alternative names
4 In Fiction
5 References
6 External links

[edit] Strategy

An example of a game.

To win or avoid losing in tic-tac-toe requires that the player consistently performs as many of the following actions as possible with each mark — listed in order of priority — without sacrificing the higher priorities:

Complete three in a row.
Block their opponent from completing three in a row.
Threaten a win with two possible completions in two rows.
Avoid a configuration in which the opponent can force the win.
Threaten a win with a possible completion (two in a row).
As second player: Get the center space, if it is not already taken.
Prevent the opponent from getting two in a row.

To be successful, the player must seek to complete as many of these objectives as possible with a single mark, without sacrificing higher priorities. The player must also think ahead to see whether a mark can be made this turn that will allow him or her to achieve a higher priority in the next turn.

The game is won or drawn after the first two marks are made, assuming perfect play for the remainder of the game. It is therefore very important for the serious tic-tac-toe player to study these openings (of which there are 12) in order to avoid making a mark that enables the opponent to force a win, or to recognize marks that can be capitalized upon in order to force the win.

The first player, whom we shall designate "X," has 3 possible positions to mark during the first turn. Superficially, it might seem that there are 9 possible positions, corresponding to the 9 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 mark. For strategy purposes, 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. The choice of which to make will depend on the player's knowledge of their opponent's weaknesses in recognizing good answers to a particular opening. In a series of games, alternating the opening mark and its superficial position can help a player win more often against a weaker player.

The second player, whom we shall designate "O," must respond to X's opening mark in such a way as to avoid the forced win. Player O must always respond to a corner opening with a center mark, and to a center opening with a corner mark. An edge opening must be answered either with a center mark, a corner mark next to the X, or an edge mark opposite the X. Any other responses will allow X to force the win. Once the opening is completed, O's task is to follow the above list of priorities in order to force the draw, or else to gain a win if X makes a weak play.

[edit] Variations

Many board games share the element of trying to be the first to get n-in-a-row: three men's morris, nine men's morris, pente, gomoku, Connect Four, Quarto, Gobblet. The m,n,k-games are a family of generalized games based on tic-tac-toe.

3-dimensional tic-tac-toe on a 3×3×3 board, though the first player has an easy win by playing in the centre. Another variant is played on a 4×4×4 board though it was solved by Victor Allis in 1994 (the first player can force a win). A more complex variant can be played on boards utilising higher dimensional space, most commonly 4 dimensions in a 3×3×3×3 board. In such games the aim is to fill up the board and get more rows of three in total than the other player.
4 player 3-dimensional tic-tac-toe: Not as easy to win by the first player, the symbols are O's X's triangles and squares, otherwise the rules to 3-D tic-tac-toe apply.
In misère tic-tac-toe you win if the other player gets n in a row. The 3×3 game is a draw.
In nine board tic-tac-toe nine tic-tac-toe boards are themselves arranged in a 3×3 grid. The first player's move may go on any board; all moves afterwards are placed in the empty spaces on the board corresponding to the square of the previous move (that is, if a move were in the upper-left square of a board, the next move would take place on the upper-left board). If a player can't move because the indicated board is full, the next move may go on any board. Victory is attained by getting 3 in a row on any board. This makes the game considerably longer and more involved than tic-tac-toe, with a definite opening, middle game and endgame.
In Tic-Tac-Chess, players play a game of chess and tic-tac-toe simultaneously. When a player captures an opponent's piece, the player can make a play on the tic-tac-toe board regardless if the other player has not yet made a play. The first person to get 3 X's or O's in a row wins the game. This makes for a much more defensive game of chess.
There is a game which is isomorphic to tic-tac-toe, but on the surface appears completely different. Players take it in turn to say a number between one and nine. A particular number may not be repeated. Both players aim to say three numbers which add up to 15. Plotting these numbers on a 3×3 magic square will reveal the exact correspondence with the game of tic-tac-toe, given that three numbers will be arranged in a straight line if and only if they add up to 15.
Two players fill out a 3×3 grid with numbers one through nine in order of priority. They then compare their grids and play tic-tac-toe by filling in the squares by the priority they listed before.
In the 1970s, there was a two player game made by Tri-ang Toys & Games called Check Lines, in which the board consisted of eleven holes arranged in a geometrical pattern of twelve straight lines each containing three of the holes. Each player had exactly five tokens and played in turn placing one token in any of the holes. The winner was the first player whose tokens were arranged in two lines of three (which by definition were intersecting lines). If neither player had won by the tenth turn, subsequent turns consisted of moving one of one's own tokens to the remaining empty hole, with the constraint that this move could only be from an adjacent hole.
Toss Across is a tic-tac-toe game where players throw bean bags at a large board to mark squares.
Jätkänshakki is a Finnish variant of the game that's played on (unlimited/limited by size) square grid with the goal of getting 5 (or more) noughts or crosses in a row. It's a very popular pastime in schools during (and off) classes.
Various game shows have been based around the game:
- Hollywood Squares is better known as celebrity tic-tac-toe.
- Secret "X" contestants play tic-tac-toe "Price is Right-Style" with Xs only.
- In Tic-Tac-Dough players put symbols up on the board by answering questions.
- In Beat the Teacher contestants answer questions to win a turn to influence a tic-tac-toe grid.
The object of the fictional D'ni game of Gemedet is to get six balls in-a-row in a 9×9×9 cube grid.
Some children play where getting a Y formation also counts as a win.
Quantum tic tac toe allows players to place a quantum superposition of numbers on the board

[edit] Alternative names

Around the world, the game has a number of alternative names.

Tic-tac-toe, tick-tat-toe, or tit-tat-toe (USA)
Noughts and crosses or naughts and crosses (United Kingdom, Ireland, Australia, New Zealand)
Xs and Os (English - United Kingdom and Ireland)
Exy-Ozys (Northern Ireland)
Boxin' OXen (Southern Ireland)
Jogo da Velha (Brazilian Portuguese)
Tris, filetto, crocetta e pallino, cerchi e croci, or tria (Italian)
Kolko i krzyzyk (A circle and a cross - Poland)
Boter, kaas en eieren(Butter, cheese and eggs) - Netherlands
Morpion France
Tres en raya Spain

Sometimes, the names of the games Tic-tac-toe (where players keep adding "pieces") and Three Men's Morris (where pieces start to move after a certain number have been placed) are confused.

[edit] In Fiction

Spoiler warning: Plot and/or ending details follow.

In the cult 1983 film WarGames tic-tac-toe is used as an allegory for nuclear war. In the film a computer hacker (Matthew Broderick) in the cold war era breaks into the missile defense computer WOPR designed to orchestrate nuclear war against Russia. In the process, he inadvertently triggers the system into "DEFCON 1" mode, whereby the computer arms its missiles in preparation for launch. The hacker eventually influences the computer to play tic-tac-toe against itself, whereby the computer determines that neither side can win – an analogy to full scale nuclear war which was made explicit when the computer then fails to find a winning stratagem for a nuclear strike.

"The best way to win is not to play at all"