Play this simple game anywhere. Take turns marking spaces to build 3 in a row. Block your opponent while creating your own winning line.

Rules

• Players choose how to mark the squares they claim. Typically one player marks squares as Xs and the other as Os. It's also possible to use two colors of game pieces, or other methods.
• Players alternate turns, attempting to claim three squares in a row, either vertically, horizontally, or diagonally.
• On each turn, a player claims any one of the nine spaces which has not already been claimed.
• When either player gets three in a row, they have won and the game ends.
• If all nine spaces are filled, the game ends in a draw, also known as a 'cat game'.

Components

Tic-Tac-Toe is simple enough that it can be played with only a pencil and paper. When playing with game pieces, the components are:

• 5 pieces for O
• 5 pieces for X
• 3x3 game board

Print

• Pen and paper version
• Single page game board and 1-inch pieces
• Single page black & white game board with 1-inch pieces for less ink usage

Buy

Game pieces:

• 1-inch wooden checkers
• Half-inch wooden pawns

Durable sets:

• Wooden set
• Marble set

History

Tic-Tac-Toe descends from a much older family of games played on 3 by 3 boards. These nine hole boards have been found in Kurna, Egypt, carved onto roofing tiles dating tentatively to 1400 BCE. Three Men's Morris was played across the Roman Empire on similar boards. These games were all played by placing stones, marbles, or other pieces onto the boards, rather than marking them. Typically, they were played with limited numbers of pieces that were placed and then moved around the board.

Tic-Tac-Toe moved onto its distinctive square board late in its development. In medieval Spain, Chess became popular and older games with their own boards moved onto grids. Thus, Alquerque lost its distinctive board and became Checkers. Tic-Tac-Toe can still be played with pieces set onto a static game board, but the common form as a pen and paper developed from several technological advances that are so old they may not quite seem technological at all. China developed paper made from mulberry bark, and around 100 CE Cai Lun presented the emperor with a process for making a mass-producible, cheap, light writing medium. In Baghdad where trees were limited, this evolved into Arabic paper made from even cheaper cloth scraps of linen. In Europe, the paper mills were erected along rivers where the flow of the water churned the raw materials into pulp. Finally in Cumbria, England, a vast deposit of pure graphite was discovered in the sixteenth century that was cut into stick rods for the first modern pencils. These allowed it to become a common game for schoolchildren. Originally called Noughts and Crosses, it also took the name of another pen-and-paper game that fell out of fashion, Tic-Tack-Toe.

The simple rules and limited choices made it an early subject for computer programming. One of the first video games, OXO (1952) by Sandy Douglas, was Tic-Tac-Toe played against a computer opponent on the massive EDSAC computer at Cambridge. In 1961, AI researchers Donald Michie and Roger Chambers built Machine Educable Noughts and Crosses Engine (MENACE), a mechanical computer built entirely from matchboxes and beads of nine colors. MENACE was capable of "learning" to play a perfect game. In 1975, MIT students built a more complex mechanical computer from tinker toys, and demonstrated its power by having it play perfect games of Tic-Tac-Toe.

Strategy

A perfect game, or a game where each player makes the best available move on each turn, will always result in a draw. In their 1972 book Human Problem Solving, psychologists Allen Newell and Herbert A. Simon compared human working memory to emerging computers, and created a list of rules by which a player could play a perfect game. Using rules ordering, a player or computer program will follow whichever rule is matched first:

1. Win: If you can complete a three-in-a-row line, do it.
2. Block: If your opponent could claim a blank space on their next turn to get three-in-a-row, block them by claiming that blank space.
3. Fork: Claim a space along two intersecting lines where you have claimed one space and your opponent none. This will give you two ways to win on your next turn and therefore make it impossible for your opponent to block you.
4. Block fork: If your opponent could create a fork on their next turn, block the fork. If possible block it in such a way that will give you two-in-a-row, forcing them to block on their next turn.
5. Center: Claim the center space if open.
6. Opposite corner: Claim the opposite corner from one your opponent has already claimed, if it's open.
7. Empty corner: Claim an empty corner, if any are open.
8. Empty side: Claim an empty side space, if any are open.

Related

• Three Men's Morris: played on the same grid using three pieces only that can be moved after all are placed.
• Gomoku: five in a row on a larger board.
• Four in a Row: a simpler game than Gomoku, Four in a Row requires a physical board but also has a satisfying sensory aspect.
• Dodgem: another game that can be played on the same 3x3 game board.