How to play Gomoku well

An open four cannot be stopped

When four of your stones are in a line and both ends are empty, blocking one end still leaves the other, and five follows. This is called an open four, and once you have one the game is yours. So Gomoku is won by the side that makes an open four first, and you must stop the opponent's before it gets there.

Block an open three when you see it

A three with both ends open becomes an open four on the next move. So the standard play is to block the opponent's open three before it becomes four. Conversely, if you make two open threes at once the opponent can only block one, and that fork is how games of Gomoku are won.

Attacking alone loses

If you only think about extending your own line, you miss the moment the opponent connects four. We measured it in our simulation. The side that never blocked and only extended its own line played the side that watched both attack and defense, and the attack-only side lost. Every move has to extend your line and watch the opponent's threat at the same time.

The center is worth more

A stone in the middle can aim for five in all four directions: row, column and both diagonals. Toward the edge there are fewer directions to grow, so when two squares are otherwise equal, the one nearer the center is better. The bot also prefers the square nearer the center when the values are equal.

How to choose the day's rule

Three of the 6 candidate rules are drawn for the day. You ban one and pick one. Each rule changes the feel of the game quite a lot.

• No center The nine squares in the middle of the board are off limits.
• No diagonals Five on a diagonal does not win. Only rows and columns count.
• Touching only You may only play next to a stone already on the board (the first move can go anywhere).
• Keep apart You may not play in the eight squares next to your own stones.
• Edge first If there is a legal square on the edge of the board, you must play there first.
• Even squares You may only play where the row number plus the column number is even.

Think about the ban first. Removing the one of the three that hurts you most is worth more than the pick. For example, No diagonals wipes out every line you were building on a diagonal, so if diagonals are your strength, ban it first.

Same day, same board

There is no randomness. The candidates are set by the date, and the opponent always plays the same moves on the same board. So if you replay a lost game you can see exactly where it turned. A loss is not bad luck but the wrong order of moves, and memorizing it makes you better.

No rule ever leaves you without a legal square. A narrowing rule that blocks the whole board would be an accident, not a rule, so if narrowing would leave nothing, it is not applied.

About the name

Omok is the Korean name for Gomoku, an old traditional game that belongs to no company. The format is common property and anyone may build one. Only the layer we added, the day's rule on top, is ours, and it was written by us rather than carried over from someone else's game.