Even Threat in Connect Four

An even threat is a threat whose winning square is located on row 2, row 4, or row 6 of the board (counting rows from the bottom up). These threats are strategically tied to player 2 because of how Connect Four's turn order interacts with the gravity-stacking mechanic. As the board fills and players are forced into specific columns, player 2 ends up naturally claiming squares on even rows, while player 1 claims squares on odd rows.

The reason this works lies in counting. Each column has 6 rows. To reach an even row in a column, an even number of pieces must already be stacked below it. If row 2 is the target, exactly 1 piece must already sit in row 1 (so the next piece lands in row 2). To reach row 4, exactly 3 pieces must sit below. To reach row 6, exactly 5 pieces must sit below. In each case, the supporting stack has an odd count, which means an odd-numbered move filled the most recent slot. Therefore the next move (an even-numbered turn, which belongs to player 2) lands on the even row.

This parity is decisive in zugzwang endgames. When both players are running out of safe columns, the player whose threats sit on rows matching their parity will win. Player 2 with multiple even threats simply waits while player 1 is forced to fill columns. Each forced fill brings player 2's even-row squares closer. Eventually player 2 plays into a column and lands directly on a four-in-a-row completion. Meanwhile player 1's pieces sit on odd rows that do not complete a winning line.

If you are player 2, plan your threats with even rows in mind. Diagonal threats are the most flexible because they cross multiple rows. By choosing diagonals that land their winning squares on row 2, 4, or 6, you stack the endgame parity in your favor. Player 1 will struggle to refute these threats once the board fills. The earliest opportunity to think about even-threat strategy is the middlegame, when threat construction begins to matter.