Four in a Row — Strategy

Odd and even rows

This topic is about how a game is decided once there are no immediate threats left and the board is filling up. The rule was found by Victor Allis, who solved the classic game in 1988.

On a 7×6 board every column has six cells — an even number. If both players simply fill the columns to the top, the first player gets the cells of the odd rows (1, 3, 5) and the second player those of the even rows (2, 4, 6): the second player can always answer in the same column the first one played.

Hence the threat rule:

• The first player (red in the first game of the match) benefits from threats on odd rows, above all on rows three and five.
• The second player benefits from threats on even rows.

A threat of "your" parity ripens by itself at the end of the game: the opponent will be forced to play under it. A threat of the "wrong" parity is usually useless — the opponent gets the turn there and blocks it.

The second player can follow the strategy of answering in the same column (called follow-up) and block whatever the first player builds, as long as the first player has no odd threat. So the first player tries to create an odd threat as early as possible, and the second tries to prevent it.

This rule is for a plain board of even height. On boards of odd height, with obstacles and on the cylinder, parity is distributed differently, and you have to count afresh.