Odd-Even Strategy in Connect Four

The odd-even strategy is one of the deepest concepts in Connect Four. It governs the endgame and determines who wins in many close positions. The core idea is straightforward. Player 1 moves first, third, fifth, and so on (odd turns). Player 2 moves second, fourth, sixth, and so on (even turns). This means that when the board fills up, player 1 will fill the last odd-numbered empty square, and player 2 will fill the last even-numbered one.

Rows in Connect Four are numbered 1 through 6 from bottom to top. A threat on an odd row (1, 3, or 5) tends to favor player 1. A threat on an even row (2, 4, or 6) tends to favor player 2. This is because the turn order determines who will be forced to play into which row as the board fills. If player 2 has a threat on an even row and the game reaches a state where both players are filling columns, player 2 will naturally claim that even-row square at the right moment.

This matters most in the endgame when most columns are nearly full. At that point, players have fewer choices. The board is almost full and each remaining move is practically forced. Whoever has threats on their "matching" rows (odd for player 1, even for player 2) holds the advantage. The opponent cannot avoid triggering those threats because they must play somewhere.

Practically, this means you should aim to build threats on rows that match your player number. If you are player 1, develop threats on rows 1, 3, and 5. If you are player 2, focus on rows 2, 4, and 6. During the midgame, consider where your threats will land. A beautiful setup on the wrong row might actually help your opponent. Counting parity correctly requires practice, but once you internalize it, you will see endgames in a completely new light. Many positions that look equal are actually winning for one side due to row parity alone.