Connect Four

Connect Four is a two-player connection game in which players take turns dropping colored discs into a vertical grid with 7 columns and 6 rows. The discs fall to the lowest available position within the chosen column, obeying gravity. The first player to form a horizontal, vertical, or diagonal line of four of their own discs wins. If the board fills completely without four in a row for either player, the game is a draw.

Rules

1. The board is a vertical grid of 7 columns and 6 rows (42 cells total).
2. Two players alternate turns. Each player has discs of their own color (traditionally red and yellow).
3. On each turn, a player chooses a column and drops one disc into it. The disc falls to the lowest empty cell in that column.
4. If a column is completely full, no disc can be placed there.
5. The first player to connect four of their discs in a line — horizontally, vertically, or diagonally — wins.
6. If all 42 cells are filled without a winner, the game is a draw.

Strategies

• Control the Center Column: The center column (column 4) is the most powerful position because it participates in the most possible four-in-a-row combinations. Strong players compete for center control early.
• Build from the Bottom: Since discs obey gravity, you must think vertically. You cannot place a disc at a specific height without first filling the cells below it. This constraint drives much of the strategy.
• Odd/Even Strategy: For the first player, controlling cells in odd-numbered rows (1, 3, 5 counting from bottom) is advantageous. The second player benefits from even rows. This arises because the first player makes moves on turns 1, 3, 5... (odd) and the second on turns 2, 4, 6... (even).
• Create Multiple Threats: Set up positions where you threaten to win in two places simultaneously. Since the opponent can only block one column per turn, a double threat guarantees victory.
• Zugzwang Awareness: Sometimes making a move is disadvantageous because it gives your opponent access to a critical cell above. Recognizing when a move would help your opponent is crucial.
• Threat Above Threat: A threat is only useful if you can actually reach the winning cell. A winning cell in row 5 is useless if filling the cells below it would give your opponent a win first. Stack your threats carefully.

Connect Four was first sold under the famous name by Milton Bradley (now part of Hasbro) in 1974. However, the concept of a vertical four-in-a-row game predates the commercial product. Captain's Mistress, a similar game, is believed to have been played by sailors as early as the 18th century — legend has it that Captain James Cook was particularly fond of it, hence the name.

The game was independently invented by Howard Wexler and Ned Strongin, who licensed it to Milton Bradley. It became one of the most successful board games of the 1970s and has remained a best-seller for decades. The physical design — a vertical plastic frame with slots for dropping discs — is both elegant and iconic, making it instantly recognizable.

Connect Four was mathematically solved in 1988 by Victor Allis in his master's thesis at Vrije Universiteit Amsterdam. He proved that the first player can always force a win with perfect play, starting from the center column. James D. Allen independently solved it the same year. The solution demonstrates that the first player wins in at most 41 moves (one cell left empty) with optimal play. Despite being solved, the game remains challenging and enjoyable for human players because perfect play requires considering thousands of positions that are beyond casual memorization.

The game has spawned many variants, including Connect Four 3D (played on a 4x4x4 vertical grid), Pop Out (where players can remove their own bottom disc), and Five in a Row (on larger boards). Connect Four is widely used in AI education as an intermediate-complexity game — harder than Tic-Tac-Toe but simpler than Chess — ideal for teaching minimax algorithms with alpha-beta pruning.