How to play Gomoku

Gomoku is five in a row on a fifteen-by-fifteen board. Both players know all the rules before the first move, which is why it is such a good game: nothing is hidden behind a rulebook, and everything is hidden in plain sight on the board.

The move

Stones are placed on the intersections of the grid, not in the squares. You play teal and move first; the computer plays magenta. Take turns placing one stone on any empty intersection. Stones never move once placed, and are never captured or removed.

Winning a game of Gomoku

The first player to make five of their own stones in an unbroken line wins — horizontally, vertically, or along either diagonal.

This is free-style Gomoku. There are no forbidden opening shapes, no restrictions on the first player, and an overline of six or more in a row counts as a win rather than being disallowed. That matters because in some tournament rulesets an overline is explicitly not a win for the first player, and beginners who learned those rules expect a different result here.

The one shape that decides the game

Everything in Gomoku follows from the open three: three of your stones in a row with an empty, playable point at each end.

Left alone for one move, an open three becomes an open four — four in a row with an empty point at both ends. An open four cannot be blocked. There are two points that complete it, your opponent has one move, so the game is already over.

This gives the whole game its rhythm. An open three must be answered immediately, every time, or you have lost. Everything else is negotiable; that is not.

A closed three — three in a row with one end blocked by an enemy stone or the board edge — is far less dangerous, because completing it makes a four with only one open end, which your opponent simply blocks. Learning to tell an open three from a closed one at a glance is most of the skill of the game.

Double threats

Since a single open three can always be answered, strong play means making two threats with one stone. The classic is the double three: a stone that completes two separate open threes at once. Your opponent can block one, and the other becomes an open four.

A four-and-three — one stone creating both a four and an open three — is even stronger, because the four must be blocked immediately, which gives you a free move to convert the three.

Almost every game between players who know what they are doing is a race to build a double threat while stopping the other person's.

Defence

Blocking is not passive. The best block is the one that also builds something of your own, so before you answer a three, check whether one of the two blocking points also extends a line you already have. A block that develops is worth two moves.

Watch for shapes that are not literally in a row: a "broken three" with a gap in it — stone, stone, gap, stone — is one move from a four and beginners routinely ignore it.

The opponent

The computer scores every possible five-in-a-row window on the board, so an open shape naturally counts for more than a blocked one, and it searches ahead with alpha-beta pruning and a transposition table. Easy looks barely a move ahead and will miss a plan. Medium punishes a loose three. Hard defends while building threats of its own.