Last updated

Tic tac toe.svg
A completed game of tic-tac-toe
Other names
  • Noughts and Crosses
  • Xs and Os
Genres Paper-and-pencil game
Setup timeMinimal
Playing time~1 minute
Skills Strategy, tactics, observation

Tic-tac-toe (American English), noughts and crosses (Commonwealth English), or Xs and Os (Canadian or Irish English) is a paper-and-pencil game for two players who take turns marking the spaces in a three-by-three grid with X or O. The player who succeeds in placing three of their marks in a horizontal, vertical, or diagonal row is the winner. It is a solved game, with a forced draw assuming best play from both players.



In American English, the game is known as "tic-tac-toe". It may also be spelled "tick-tack-toe", "tick-tat-toe", or "tit-tat-toe". [1] [2]

In Commonwealth English (particularly British, South African, Indian, Australian, and New Zealand English), the game is known as "noughts and crosses", alternatively spelled "naughts and crosses". This name derives from the shape of the marks in the game (i.e the X and O); "nought" is an older name for the number zero, while "cross" refers to the X shape. While the term "nought" is now less commonly used, the name "noughts and crosses" is still preferred over the American name "tic-tac-toe" in these countries.

Sometimes, 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 with each other.


Tic-tac-toe is played on a three-by-three grid by two players, who alternately place the marks X and O in one of the nine spaces in the grid.

In the following example, the first player (X) wins the game in seven steps:

Game of Tic-tac-toe, won by X Tic-tac-toe-game-1.svg
Game of Tic-tac-toe, won by X

There is no universally agreed rule as to who plays first, but in this article the convention that X plays first is used.

Players soon discover that the best play from both parties leads to a draw. Hence, tic-tac-toe is often played by young children who may not have discovered the optimal strategy.

Because of the simplicity of tic-tac-toe, it is often used as a pedagogical tool for teaching the concepts of good sportsmanship 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 or to enumerate the 765 essentially different positions (the state space complexity) or the 26,830 possible games up to rotations and reflections (the game tree complexity) on this space. [3] If played optimally by both players, the game always ends in a draw, making tic-tac-toe a futile game. [4]

Incidence structure for tic-tac-toe Three Men's Morris variant board.svg
Incidence structure for tic-tac-toe

The game can be generalized to an m,n,k-game, in which two players alternate placing stones of their own color on an m-by-n board with the goal of getting k of their own color in a row. Tic-tac-toe is the 3,3,3-game. [5] Harary's generalized tic-tac-toe is an even broader generalization of tic-tac-toe. It can also be generalized as an nd game, specifically one in which n = 3 and d = 2. [6] It can be generalised even further by playing on an arbitrary incidence structure, where rows are lines and cells are points. Tic-tac-toe's incidence structure consists of nine points, three horizontal lines, three vertical lines, and two diagonal lines, with each line consisting of at least three points.


Games played on three-in-a-row boards can be traced back to ancient Egypt, [7] where such game boards have been found on roofing tiles dating from around 1300 BC. [8]

An early variation of tic-tac-toe was played in the Roman Empire, around the first century BC. It was called terni lapilli (three pebbles at a time) and instead of having any number of pieces, each player had only three; thus, they had to move them around to empty spaces to keep playing. [9] The game's grid markings have been found chalked all over Rome. Another closely related ancient game is three men's morris which is also played on a simple grid and requires three pieces in a row to finish, [10] and Picaria, a game of the Puebloans.

The different names of the game are more recent. The first print reference to "noughts and crosses" (nought being an alternative word for 'zero'), the British name, appeared in 1858, in an issue of Notes and Queries . [11] The first print reference to a game called "tick-tack-toe" occurred in 1884, but referred to "a children's game played on a slate, consisting of trying with the eyes shut to bring the pencil down on one of the numbers of a set, the number hit being scored".[ This quote needs a citation ] "Tic-tac-toe" may also derive from "tick-tack", the name of an old version of backgammon first described in 1558. The US renaming of "noughts and crosses" to "tic-tac-toe" occurred in the 20th century. [12]

In 1952, OXO (or Noughts and Crosses), developed by British computer scientist Sandy Douglas for the EDSAC computer at the University of Cambridge, became one of the first known video games. [13] [14] The computer player could play perfect games of tic-tac-toe against a human opponent. [13]

In 1975, tic-tac-toe was also used by MIT students to demonstrate the computational power of Tinkertoy elements. The Tinkertoy computer, made out of (almost) only Tinkertoys, is able to play tic-tac-toe perfectly. [15] It is currently on display at the Computer History Museum. [16]


When considering only the state of the board, and after taking into account board symmetries (i.e. rotations and reflections), there are only 138 terminal board positions. A combinatorics study of the game shows that when "X" makes the first move every time, the game outcomes are as follows: [17]


Optimal strategy for player X if starting in upper left. In each grid, the shaded red X denotes the optimal move, and the location of O's next move gives the next subgrid to examine. Only two sequences of moves by O (both starting with the center, top-right, left-mid) lead to a draw, with the remaining sequences leading to wins from X. Tictactoe-X.svg
Optimal strategy for player X if starting in upper left. In each grid, the shaded red X denotes the optimal move, and the location of O's next move gives the next subgrid to examine. Only two sequences of moves by O (both starting with the center, top-right, left-mid) lead to a draw, with the remaining sequences leading to wins from X.
Optimal strategy for player O. Player O can only force a win or draw by playing in the center first. Tictactoe-O.svg
Optimal strategy for player O. Player O can only force a win or draw by playing in the center first.

A player can play a perfect game of tic-tac-toe (to win or at least draw) if, each time it is their turn to play, they choose the first available move from the following list, as used in Newell and Simon's 1972 tic-tac-toe program. [19]

  1. Win: If the player has two in a row, they can place a third to get three in a row.
  2. Block: If the opponent has two in a row, the player must play the third themselves to block the opponent.
  3. Fork: Cause a scenario where the player has two ways to win (two non-blocked lines of 2).
  4. 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.)
  5. 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.)
  6. Opposite corner: If the opponent is in the corner, the player plays the opposite corner.
  7. Empty corner: The player plays in a corner square.
  8. 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. [20] This might suggest that the corner is the best opening move for X, however another study [21] shows that if the players are not perfect, an opening move in the center is best for X.

The second player, who shall be designated "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.

More detailed, to guarantee a draw, O should adopt the following strategies:

When X plays corner first, and O is not a perfect player, the following may happen:

Further details

Consider a board with the nine positions numbered as follows:


When X plays 1 as their opening move, then O should take 5. Then X takes 9 (in this situation, O should not take 3 or 7, O should take 2, 4, 6 or 8):

or 6 (in this situation, O should not take 4 or 7, O should take 2, 3, 8 or 9. In fact, taking 9 is the best move, since a non-perfect player X may take 4, then O can take 7 to win).

In both of these situations (X takes 9 or 6 as the second move), X has a 1/3 property to win.

If X is not a perfect player, X may take 2 or 3 as a second move. Then this game will be a draw, X cannot win.

If X plays 1 opening move, and O is not a perfect player, the following may happen:

Although O takes the only good position (5) as the first move, O takes a bad position as the second move:

Although O takes good positions in the first two moves, O takes a bad position in the third move:

O takes a bad position as first move (except of 5, all other positions are bad):


Many board games share the element of trying to be the first to get n-in-a-row, including three men's morris, nine men's morris, pente, gomoku, Qubic, Connect Four, Quarto, Gobblet, Order and Chaos, Toss Across, and Mojo. Tic-tac-toe is an instance of an m,n,k-game, where two players alternate taking turns on an m×n board until one of them gets k in a row. Harary's generalized tic-tac-toe is an even broader generalization. The game can be generalised even further by playing on an arbitrary hypergraph, where rows are hyperedges and cells are vertices.

Other variations of tic-tac-toe include:

One can play on a board of 4x4 squares, winning in several ways. Winning can include: 4 in a straight line, 4 in a diagonal line, 4 in a diamond, or 4 to make a square.

Another variant, Qubic, is played on a 4×4×4 board; it was solved by Oren Patashnik in 1980 (the first player can force a win). [22] Higher dimensional variations are also possible. [6]

fearortry r
beboatby b
tenonany n







Various game shows have been based on tic-tac-toe and its variants:[ citation needed ]

See also

Related Research Articles

Three men's morris is an abstract strategy game played on a three by three board that is similar to tic-tac-toe. It is also related to six men's morris and nine men's morris. A player wins by forming a mill, that is, three of their own pieces in a row.

<span class="mw-page-title-main">3D tic-tac-toe</span> 1978 video game

3D tic-tac-toe, also known by the trade name Qubic, is an abstract strategy board game, generally for two players. It is similar in concept to traditional tic-tac-toe but is played in a cubical array of cells, usually 4×4×4. Players take turns placing their markers in blank cells in the array. The first player to achieve four of their own markers in a row wins. The winning row can be horizontal, vertical, or diagonal on a single board as in regular tic-tac-toe, or vertically in a column, or a diagonal line through four boards.

In mathematics, the Hales–Jewett theorem is a fundamental combinatorial result of Ramsey theory named after Alfred W. Hales and Robert I. Jewett, concerning the degree to which high-dimensional objects must necessarily exhibit some combinatorial structure; it is impossible for such objects to be "completely random".

In combinatorial game theory, the strategy-stealing argument is a general argument that shows, for many two-player games, that the second player cannot have a guaranteed winning strategy. The strategy-stealing argument applies to any symmetric game in which an extra move can never be a disadvantage. A key property of a strategy-stealing argument is that it proves that the first player can win the game without actually constructing such a strategy. So, although it might prove the existence of a winning strategy, the proof gives no information about what that strategy is.

<span class="mw-page-title-main">Toss Across</span> Game similar to tic-tac-toe

Toss Across is a game first introduced in 1969 by the now defunct Ideal Toy Company. The game was designed by Marvin Glass and Associates and created by Hank Kramer, Larry Reiner and Walter Moe, and is now distributed by Mattel. It is a game in which participants play tic-tac-toe by lobbing small beanbags at targets in an attempt to change the targets to their desired letter. As in traditional tic-tac-toe, the first player to get three of their letters in a row wins the game. There are other similar games to Toss Across known under different names, such as Tic Tac Throw.

<span class="mw-page-title-main">Quantum tic-tac-toe</span>

Quantum tic-tac-toe is a "quantum generalization" of tic-tac-toe in which the players' moves are "superpositions" of plays in the classical game. The game was invented by Allan Goff of Novatia Labs, who describes it as "a way of introducing quantum physics without mathematics", and offering "a conceptual foundation for understanding the meaning of quantum mechanics".

Harary's generalized tic-tac-toe or animal tic-tac-toe is a generalization of the game tic-tac-toe, defining the game as a race to complete a particular polyomino on a square grid of varying size, rather than being limited to "in a row" constructions. It was devised by Frank Harary in March 1977, and is a broader definition than that of an m,n,k-game.

<span class="mw-page-title-main">Nine holes</span> Abstract strategy game

Nine holes is a two-player abstract strategy game from different parts of the world and is centuries old. It was very popular in England. It is related to tic-tac-toe, but even more related to three men's morris, achi, tant fant, shisima, picaria, and dara, because pieces are moved on the board to create the 3 in a row. It is an alignment game.

<span class="mw-page-title-main">Tant fant</span>

Tant fant is a two-player abstract strategy game from India. It is related to tic-tac-toe, but more closely related to three men's morris, nine holes, achi, shisima, and dara, because pieces are moved on the board to create the 3 in a row. It is an alignment game.

Shisima is a two-player abstract strategy game from Kenya. It is related to tic-tac-toe, and even more so to three men's morris, Nine Holes, Achi, Tant Fant, and Dara, because pieces are moved on the board to create the 3-in-a-row. Unlike those other games, Shisima uses an octagonal board.

<span class="mw-page-title-main">Picaria</span> Two-player abstract strategy game from the Zuni Native American Indians or Pueblo Indians

Picaria is a two-player abstract strategy game from the Zuni Native American Indians or the Pueblo Indians of the American Southwest. It is related to tic-tac-toe, but more related to three men's morris, Nine Holes, Achi, Tant Fant, and Shisima, because pieces can be moved to create the three-in-a-row. Picaria is an alignment game.

<span class="mw-page-title-main">Zillions of Games</span> General game playing software

Zillions of Games is a commercial general game playing system developed by Jeff Mallett and Mark Lefler in 1998. The game rules are specified with S-expressions, Zillions rule language. It was designed to handle mostly abstract strategy board games or puzzles. After parsing the rules of the game, the system's artificial intelligence can automatically play one or more players. It treats puzzles as solitaire games and its AI can be used to solve them.

Tic Tac Toe may refer to:

<span class="mw-page-title-main">Ultimate tic-tac-toe</span> Variant of tic-tac-toe game

Ultimate tic-tac-toe is a board game composed of nine tic-tac-toe boards arranged in a 3 × 3 grid. Players take turns playing on the smaller tic-tac-toe boards until one of them wins on the larger board. Compared to traditional tic-tac-toe, strategy in this game is conceptually more difficult and has proven more challenging for computers.

<span class="mw-page-title-main">Notakto</span> Pen and paper game

Notakto is a tic-tac-toe variant, also known as neutral or impartial tic-tac-toe. The game is a combination of the games tic-tac-toe and Nim, played across one or several boards with both of the players playing the same piece. The game ends when all the boards contain a three-in-a-row of Xs, at which point the player to have made the last move loses the game. However, in this game, unlike tic-tac-toe, there will always be a player who wins any game of Notakto.

<span class="mw-page-title-main">Wild tic-tac-toe</span>

Wild tic-tac-toe is an impartial game similar to tic-tac-toe. However, in this game players can choose to place either X or O on each move. This game can also be played in its misere form where if a player creates a three-in-a-row of marks, that player loses the game.

<span class="mw-page-title-main">Tic-tac-toe variants</span> Overview about tic-tac-toe variants

Tic-tac-toe is an instance of an m,n,k-game, where two players alternate taking turns on an m×n board until one of them gets k in a row. Harary's generalized tic-tac-toe is an even broader generalization. The game can also be generalized as a nd game. The game can be generalised even further from the above variants by playing on an arbitrary hypergraph where rows are hyperedges and cells are vertices.

A nd game (or nk game) is a generalization of the combinatorial game tic-tac-toe to higher dimensions. It is a game played on a nd hypercube with 2 players. If one player creates a line of length n of their symbol (X or O) they win the game. However, if all nd spaces are filled then the game is a draw. Tic-tac-toe is the game where n equals 3 and d equals 2 (3, 2). Qubic is the (4, 3) game. The (n > 0, 0) or (1, 1) games are trivially won by the first player as there is only one space (n0 = 1 and 11 = 1). A game with d = 1 and n > 1 cannot be won if both players are playing well as an opponent's piece will block the one-dimensional line.

A strong positional game is a kind of positional game. Like most positional games, it is described by its set of positions and its family of winning-sets. It is played by two players, called First and Second, who alternately take previously untaken positions.

<span class="mw-page-title-main">Matchbox Educable Noughts and Crosses Engine</span> Mechanical computer made of matchboxes

The Matchbox Educable Noughts and Crosses Engine was a mechanical computer made from 304 matchboxes designed and built by artificial intelligence researcher Donald Michie in 1961. It was designed to play human opponents in games of noughts and crosses (tic-tac-toe) by returning a move for any given state of play and to refine its strategy through reinforcement learning.


  1. Garcia, Dan. "GamesCrafters: Tic-Tac-Toe". gamescrafters.berkeley.edu. Retrieved June 8, 2021.
  2. "The History of Tic Tac Toe and Where it is Now". Aurosi. July 1, 2019. Retrieved June 8, 2021.
  3. Schaefer, Steve (2002). "MathRec Solutions (Tic-Tac-Toe)". Mathematical Recreations. Archived from the original on June 28, 2013. Retrieved September 18, 2015.
  4. W., Weisstein, Eric. "Tic-Tac-Toe". mathworld.wolfram.com. Retrieved May 12, 2017.{{cite web}}: CS1 maint: multiple names: authors list (link)
  5. Pham, Duc-Nghia; Park, Seong-Bae (November 12, 2014). PRICAI 2014: Trends in Artificial Intelligence: 13th Pacific Rim International Conference on Artificial Intelligence. Springer. p. 735. ISBN   978-3-319-13560-1.
  6. 1 2 3 Golomb, Solomon W.; Hales, Alfred W. (2002). "Hypercube tic-tac-toe" (PDF). More Games of No Chance (Berkeley, CA, 2000). Math. Sci. Res. Inst. Publ. 42. Cambridge Univ. Press: 167–182. MR   1973012. Archived (PDF) from the original on February 6, 2011.
  7. Zaslavsky, Claudia (1982). Tic Tac Toe: And Other Three-In-A Row Games from Ancient Egypt to the Modern Computer . Crowell. ISBN   0-690-04316-3.
  8. Parker, Marla (1995). She Does Math!: Real-life Problems from Women on the Job. Mathematical Association of America. p. 153. ISBN   978-0-88385-702-1.
  9. "Tic tac toe Ancient Roman 1st century BC". Sweetooth Design Company. Retrieved December 4, 2016.
  10. "Morris Games". www-cs.canisius.edu. Archived from the original on March 13, 2013. Retrieved September 5, 2012.
  11. Notes and Queries  . Series 2. Vol. VI. p. 152 via Wikisource. [ scan   Wikisource-logo.svg ]
  12. Oxford English Dictionary entries for "Noughts and Crosses", "Tick-Tack" and "Tick-Tack-Toe", dictionary.oed.com
  13. 1 2 Wolf, Mark J. P. (August 16, 2012). Encyclopedia of Video Games: The Culture, Technology, and Art of Gaming. Greenwood Publishing Group. pp. 3–7. ISBN   978-0-313-37936-9.
  14. Cohen, D.S. (March 12, 2019). "OXO aka Noughts and Crosses". Lifewire. Retrieved August 29, 2019.
  15. "Tinkertoys and tic-tac-toe". Archived from the original on August 24, 2007. Retrieved September 27, 2007.
  16. Original Tinkertoy Computer. January 5, 1978.
  17. Bolon, Thomas (2013). How to never lose at Tic-Tac-Toe. BookCountry. p. 7. ISBN   978-1-4630-0192-6.
  18. Delinski, Bernie (January 21, 2014). "Searching for the cat in tic tac toe". timesdaily.com. Times Daily.
  19. Kevin Crowley, Robert S. Siegler (1993). "Flexible Strategy Use in Young Children's Tic-Tac-Toe". Cognitive Science. 17 (4): 531–561. doi: 10.1207/s15516709cog1704_3 .
  20. Gardner, Martin (1988). Hexaflexagons and Other Mathematical Diversions. University of Chicago Press. ISBN   978-0-226-28254-1.
  21. Kutschera, Ant (April 7, 2018). "The best opening move in a game of tic-tac-toe". The Kitchen in the Zoo. Retrieved August 29, 2019.
  22. Patashnik, Oren (September 1, 1980). "Qubic: 4 × 4 × 4 Tic-Tac-Toe". Mathematics Magazine. 53 (4): 202–216. doi:10.2307/2689613. ISSN   0025-570X. JSTOR   2689613.
  23. Averbach, Bonnie; Chein, Orin (2000). Problem Solving Through Recreational Mathematics. Dover Publications. p.  252. ISBN   978-0-486-40917-7.
  24. Mendelson, Elliott (2016). Introducing Game Theory and its Applications. CRC Press. p. 19. ISBN   978-1-4822-8587-1.
  25. "Wild Tic-Tac-Toe". Puzzles in Education. December 11, 2007. Retrieved August 29, 2019.
  26. Epstein, Richard A. (December 28, 2012). The Theory of Gambling and Statistical Logic. Academic Press. p. 450. ISBN   978-0-12-397870-7.
  27. 1 2 3 Juul, Jesper (2011). Half-Real: Video Games Between Real Rules and Fictional Worlds. MIT Press. p. 51. ISBN   978-0-262-51651-8.
  28. Michon, John A. (January 1, 1967). "The Game of JAM: An Isomorph of Tic-Tac-Toe". The American Journal of Psychology. 80 (1): 137–140. doi:10.2307/1420555. JSTOR   1420555. PMID   6036351.
  29. "TicTacToe Magic" (PDF). Archived from the original (PDF) on December 20, 2016. Retrieved December 17, 2016.
  30. "Tic-Tac-Toe as a Magic Square". Oh Boy! I Get to do Math!. May 30, 2015. Retrieved August 29, 2019.
  31. Schumer, Peter D. (2004). Mathematical Journeys. John Wiley & Sons. pp. 71–72. ISBN   978-0-471-22066-4.
  32. "Check Lines". BoardGameGeek. Retrieved August 29, 2019.
  33. Twice crosses-circles
  34. Goff, Allan (November 2006). "Quantum tic-tac-toe: A teaching metaphor for superposition in quantum mechanics". American Journal of Physics. 74 (11). College Park, MD: American Association of Physics Teachers: 962–973. Bibcode:2006AmJPh..74..962G. doi:10.1119/1.2213635. ISSN   0002-9505.
  35. "Tit, tat, toe". The Library of Congress. Retrieved August 29, 2019.
  36. "452: Poultry Slam 2011". This American Life. December 2, 2011. Retrieved May 28, 2016.
  37. Trillin, Calvin (February 1, 1999). "The Chicken Vanishes". The New Yorker. ISSN   0028-792X . Retrieved August 29, 2019.
  38. "Why did the chicken win the game? Conditioning". Star Tribune. August 28, 2018. Retrieved September 15, 2019.