Gomoku

Gomoku is a traditional Japanese board game that uses a go board and two differently colored stones, but it can just as easily be played with a pencil and paper. The objective is to get five in a row, either horizontally, vertically or diagonally.

To get started, mark off a square grid with dimensions 15x15 and choose a player to go first. They will be X and the second player will be O. The first player has a slight advantage, so the following convention is used to remove that advantage. The first player makes three marks (two Xs and one O), wherever they wish. The second player is then given a choice:

1. Accept those moves and place the second O.
2. Swap places and become the first player. Their opponent then places the second O.
3. Place two more marks (one O and one X) and let the opponent choose their side.

Once this opening sequence has been completed, players take turns alternatively placing marks onto the board. The first player to get five in a row is the winner.

Example Gomoku game showing the state of the board after the first move by X. The second player has chosen option 1 above and is about to place their second O.

Strategy

Open Four - If you can get four in a row, that is not blocked at either end, this is called an open four. You are guaranteed to win with this because your opponent cannot block both ends in one turn. You can only make an open four by starting with an open three.

Open Three - An open three is any row of three that can be turned into an open four. Usually, these are three in a row, but it can also be two in a row, a blank spot and then one more mark. If your opponent doesn't block this, then you can get an open four on the next turn and win on the following turn.

Gomoku arrangements showing an open four at the top and some open threes below.

While trying to create open threes and fours yourself, it's important to watch your opponent's moves and make sure you block any attempt from them to create an open three.

Fork - One way to prevent your opponent from blocking your open threes is to make a move that produces two open threes at the same time. This is called a fork. Your opponent will only be able to block one of these, so you'll be able to turn the other one into an open four.

Placing your X in either of the highlighted spots will produce a winning fork.