**History**

An early variant of Tic-tac-toe was played in the Roman Empire, around the first century BC. It was called Terni Lapilli and instead of having any number of pieces, each player only had three, thus they had to move them around to empty spaces to keep playing. The game’s grid markings have been found chalked all over Rome. However, according to Claudia Zaslavsky’s book Tic Tac Toe: And Other Three-In-A Row Games from Ancient Egypt to the Modern Computer, Tic-tac-toe could originate back to ancient Egypt. 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.

The different names of the game are more recent . The first print reference to “Noughts and crosses”, the British name, appeared in 1864. In his novel “Can You Forgive Her”, 1864, Anthony Trollope refers to a clerk playing “tit-tat-toe”. 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 in trying with the eyes shut to bring the pencil down on one of the numbers of a set, the number hit being scored”. “Tic-tac-toe” may also derive from “tick-tack”, the name of an old version of backgammon first described in 1558. The U.S. renaming of Noughts and crosses as Tic-tac-toe occurred in the 20th century.

In 1952, OXO (or Noughts and Crosses) for the EDSAC computer became one of the first known video games. The computer player could play perfect games of Tic-tac-toe against a human opponent.

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. It is currently on display at the Museum of Science, Boston.

**Combinatorics**

The first two plies of the game tree for Tic-tac-toe. Once rotations and reflections are eliminated, there are only three opening moves – a corner, a side or the middle.

Despite its apparent simplicity, Tic-tac-toe requires detailed analysis to determine even some elementary combinatory facts, the most interesting of which are the number of possible games and the number of possible positions. A position is merely a state of the board, while a game usually refers to the way a terminal position is obtained.

Naive counting leads to 19,683 possible board layouts (39 since each of the nine spaces can be X, O or blank), and 362,880 (i.e., 9!) possible games (different sequences for placing the Xs and Os on the board). However, two matters much reduce these numbers:

• The game ends when three-in-a-row is obtained.

• The number of Xs is always either equal to or exactly 1 more than the number of Os (if X starts).

The complete analysis is further complicated by the definitions used when setting the conditions, like board symmetries.

**Number of terminal positions**

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. Assuming that X makes the first move every time:

• 91 distinct positions are won by (X)

• 44 distinct positions are won by (O)

• 3 distinct positions are drawn

**Strategy**

Optimal strategy for player X. 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. Note that only two sequences of moves by O (both starting with 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 always force a win or draw by taking center. If it is taken by X, then O must take a corner

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

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**: Create an opportunity where the player has two threats to win (two non-blocked lines of 2).

4. **Blocking an opponent’s fork:**

• Option 1: The player should create two in a row to force the opponent into defending, as long as it doesn’t result in them creating a fork. For example, if “X” has a corner, “O” has the center, and “X” has the opposite corner as well, “O” must not play a corner in order to win. (Playing a corner in this scenario creates a fork for “X” to win.)

• Option 2: If there is a configuration where the opponent can fork, the player should block that fork.

5. **Center**: A player marks the center. (If it is the first move of the game, playing on a corner gives “O” 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 4 sides.

The first player, who shall be designated “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; however, playing the corner gives the opponent the smallest choice of squares which must be played to avoid losing.

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.

To guarantee a draw for O, however:

• If X does not play center opening move (playing a corner is the best opening move), take center, and then a side middle. This will stop any forks from happening. If O plays a corner, a perfect X player has already played the corner opposite their first and proceeds to play a 3rd corner, stopping O’s 3-in-a-row and making their own fork. However, if X is not a perfect player and has played a corner and then a side middle, O should not play the opposite side middle as the second move, because then X is not forced to block in the next move and can fork.

• If O takes center (best move for them), X should take the corner opposite the original, and proceed as detailed above. The only way for O to force a tie against a perfect X player is if O plays middle and then a side-middle.

• If O plays a corner or side-middle first, X is guaranteed to win:

• If corner, X simply takes any of the other 2 corners, and then the last, a fork.

• If O plays a side-middle, X takes the only corner that O’s blocking won’t make 2 in a row. O will block, but the best of the other two will be seen by X, and O is forked.

• If X plays center opening move, O should pay attention and not allow a fork. X should play a corner first