Tic-tac-toe rules
Two players take turns writing their mark in an empty square, one crosses, the other noughts. The first to get a row of their own marks — across, down or corner to corner — wins. In Britain the same game is called noughts and crosses, and it is the one everybody has played in the margin of an exercise book.
The rules fit in a sentence, but the classic board is so small that two players who know it always draw. So besides the 3×3 game there are two bigger boards here: four in a row on 4×4, and five in a row on a 15×15 board, the way it is played across a whole page of squared paper.
The three boards
| Board | A win is | With the best play on both sides |
|---|---|---|
| 3×3 | three in a row | always a draw |
| 4×4 | four in a row | a draw too, but far easier to lose |
| 15×15 | five in a row, or more | crosses win |
The classic 3×3 board is where every table starts. Whoever sets up the game chooses the board before the first move, and the rules are the same on all three: only the length of the row changes.
Crosses go first
Crosses always move first, on every board. At a table with a friend, whoever opens it picks whether they play crosses or noughts; against the computer you pick before the game. Moving first is an advantage — small on 3×3, large on the big board — which is why the players swap marks after each game at a friend's table.
A turn
On your turn you write your mark in any empty square. There is no passing, and there is nothing else to do: no pieces to move, nothing to capture.
- A mark stays where it is put. It cannot be moved or rubbed out later.
- A square already marked, by either player, cannot be used again.
Each move has 15 seconds. If your time runs out, a mark is put down for you in a random empty square — on the big board, one near the marks already down, rather than somewhere in an empty corner. Against the computer only your own moves are timed, and if you switch to another tab the clock waits until you come back. At a friend's table, whoever opens it can turn the clock off before the start.
Winning
You win the moment you complete a row of your own marks in a straight line:
- across (a row),
- down (a column), or
- corner to corner, along either diagonal.
The winning row is struck through in red pen, and the game ends there. The row must be unbroken: a gap, or one of your opponent's marks in the middle of it, means it is not a row.
On 3×3 you need three, on 4×4 four — so on both, a winning row runs from one side of the board to the other. On 15×15 you need five, and they can be anywhere on the board.
Longer rows count
On the big board a row of six or more wins too. It often happens by filling a gap: two marks, a space, three more, and the space gets filled.
If you have played gomoku by its tournament rules, note that these rules are freer: there is no "exactly five" rule, no move that is forbidden to crosses, and no special opening. Five or more in a row, anywhere, wins.
Draws
A game is a draw when the board is full and nobody has a row. On 3×3 this is the usual ending between two careful players. On 15×15 it almost never comes to that.
At a table with a friend you can also agree a draw. Either player may offer a draw, and the other accepts or declines. Playing a move instead of answering declines it too. You can offer once per move.
Resigning
You may resign at any point. It counts as a loss. On the big board it is the honest thing to do once your opponent has an open four (four in a row with both ends free) and you have no row of your own to finish first: they cannot be stopped.
Playing a friend
Open a table and send your friend the link. While you wait, you choose the board — 3×3, 4×4 or 15×15 — and whether you play crosses or noughts. When your friend arrives, they press Ready, and you press Start. Nothing is played, and no clock runs, until then.
After a game, Play again starts the next one on the same board and keeps the score. The players swap marks, so the other one gets crosses and the first move. Change settings, which only the player who opened the table can press, goes back to the waiting room to change the board or the marks; the score is kept then too.
Playing the computer
The computer plays at three levels:
- Easy takes a win when it sees one, but plays loosely otherwise and lets many of your threats through.
- Medium wins when it can and blocks your rows, but now and then plays a weaker move.
- Hard plays its best. On 3×3 that means it never loses: the most you can get is a draw, and that takes a correct answer to every move.
There is nothing hidden in this game, so the computer sees exactly what you see: the board. You choose the board and your mark before each game. Take noughts to practise defending, and crosses to practise attacking.
Ready? Play tic-tac-toe against the computer, or read how to win at tic-tac-toe first.