Tic-Tac-Toe rules

Tic-tac-toe is a game for two players on a 3×3 grid. One player uses X, the other uses O, and they take turns placing one mark in an empty square. The first player to get three marks in a row, across, down or diagonally, wins. If the grid fills up with no line of three, the game is a draw. That is the entire rule set, and this guide covers each part in detail, plus the variants people play and the questions that start arguments at the table.

Overview

Tic-tac-toe is the American name. In the UK, Ireland, Australia and many other places it is called noughts and crosses, because O is a “nought” (zero) and X is a “cross”. The rules are the same everywhere. You need nothing more than paper and a pencil, a chalkboard, a stick in the sand or, on this site, a shared phone or tablet.

Games where players race to line up three pieces on a small grid are very old. The Roman game terni lapilli is often mentioned as an ancestor, but how directly it led to the pencil-and-paper version is not certain. What is clear is that tic-tac-toe has been a common first strategy game for children for a long time, and it is widely played around the world.

Setting up

Draw two vertical lines and two horizontal lines to make a grid of nine equal squares. The squares are usually described by position:

• Corners (1, 3, 7 and 9) are each part of three possible lines.
• The center (5) is part of four lines: the middle row, the middle column and both diagonals.
• Edges (2, 4, 6 and 8) are each part of only two lines.

Decide who plays X and who plays O, and who goes first. Then start with an empty grid.

Objective

Be the first player to place three of your own marks in a straight line. A line can be:

1. A row: three squares across (1-2-3, 4-5-6 or 7-8-9).
2. A column: three squares down (1-4-7, 2-5-8 or 3-6-9).
3. A diagonal: corner to corner through the center (1-5-9 or 3-5-7).

That makes eight possible winning lines on the board.

How a turn works

Players alternate, one mark per turn:

1. The first player places their mark in any empty square.
2. The second player places their mark in any other empty square.
3. Turns continue back and forth until someone completes a line or the board is full.

You must place a mark on your turn; there is no passing. You may only use an empty square, and you can never place a mark on top of another one.

The rules in detail

Who goes first

By tradition X goes first, but the rules don’t require it. Common options are to flip a coin, to let the youngest player start, or to let whoever lost the previous game go first. Because the first player gets more chances to build threats, alternating the start between games is the fairest habit. On this site you pick the first player in the setup, and Rematch automatically lets the other player start the next game.

Marks are permanent

Once a mark is placed, it stays for the rest of the game. You can’t move it, remove it or swap it. (On this site Undo takes back the last move, which is handy for slips, especially with younger players. Agree beforehand whether undo is allowed in a serious game.)

The game ends immediately

As soon as a player completes a line of three, the game is over and that player wins, even if empty squares remain. A move that completes a line on the ninth and final square is still a win, not a draw.

Drawn games

If all nine squares are filled and neither player has three in a row, the game is a draw. In the United States a draw is often called a cat’s game. Draws are very common between experienced players, because tic-tac-toe is a solved game: with perfect play from both sides, the result is always a draw.

Winning, drawing and keeping score

A single game ends in one of three ways: X wins, O wins, or a draw. Because games are so short, most people play a series. A simple way to keep score is to count wins for each player and the number of draws, and to alternate who starts. This site keeps that running score for you during a session: the game-over screen shows something like “Maya 3 – 2 Leo · 1 draw”.

There are 255,168 different ways a game of tic-tac-toe can play out from an empty board, counting every order of moves until someone wins or the board fills. Even so, a handful of simple ideas, covered in the tic-tac-toe strategy tutorial, are enough to never lose.

Variants and house rules

The classic game is the one described above, and it is the one this site uses: a 3×3 grid, three in a row to win, either player may start, and each rematch alternates the starting player. People also enjoy these variations:

• 4×4 tic-tac-toe. A bigger grid where you usually need four in a row. More room means more planning, and draws are frequent.
• Ultimate tic-tac-toe. Nine small tic-tac-toe boards arranged in a big 3×3 grid. The square you play in a small board decides which small board your opponent must play in next. Win small boards to claim squares on the big board.
• Reverse (misère) tic-tac-toe. The player who makes three in a row loses. It sounds simple but turns the usual strategy upside down.
• Moving-piece tic-tac-toe. Each player has only three marks or counters. Once all six are on the board, players take turns sliding one of their own pieces to an empty square. This is close to the old game Three Men’s Morris; its bigger cousin is Nine Men’s Morris.
• Gravity and bigger lines. Dropping pieces into columns and aiming for four gives you Four in a Row, while a 15×15 board with five in a row gives you Gomoku.

Common mistakes and disputes

“Diagonals don’t count.” They do. Both diagonals are standard winning lines.

“You can’t start in the center.” You can. Any empty square is legal for the first move. The center is simply a strong choice.

“I meant to go there.” Marks are permanent once placed. If you allow take-backs, agree on it before the game starts.

“It’s my turn again because I blocked you.” Blocking earns no extra turn. Players always alternate one mark at a time.

Forgetting to block. The most common way to lose is not noticing that your opponent has two in a row with an empty third square. Check the whole board before every move.

Playing the same opening every time. Against a friend that is fine, but against a smart opponent predictable openings lead to the same traps. Mixing up corners and the center keeps games interesting.

Glossary

• Mark: an X or an O placed in a square.
• Line: three squares in a row, column or diagonal. There are eight.
• Threat: two of your marks in a line with the third square empty.
• Block: placing your mark in the empty square of your opponent’s threat.
• Fork: one move that creates two threats at once, so your opponent can only block one.
• Corner, edge, center: the three types of squares (see the diagram above).
• Cat’s game: an American name for a draw.
• Solved game: a game whose result with perfect play is known. For tic-tac-toe, that result is a draw.

Teaching the game to children

Tic-tac-toe is often the first strategy game a child learns, and a few small adjustments make it more fun at that age:

• Explain the goal with your finger. Trace a row, a column and a diagonal on the empty grid so the idea of “three in a row” is concrete before the first move.
• Let them go first. Starting is a small advantage, and early wins keep a young player interested.
• Point out threats out loud. Saying “I have two in a row, where should you go?” teaches blocking far faster than winning silently.
• Play on Easy against the computer. It makes mistakes on purpose, so children can practise spotting wins and blocks without a grown-up hovering.
• Keep games short and celebrate draws. Once a child can force a draw, they have understood the whole game, and that is a real achievement.

From there, games with the same “line up your pieces” idea but more room to plan are a natural next step.

Play it now

The quickest way to learn is to play a few rounds. Open Tic-Tac-Toe to play with a friend on one device or against the computer on Easy, Medium or Hard. When you are ready to improve, the beginner strategy tutorial explains forks, the best replies to every opening and how to never lose.