The history of Connect Four

Before the box: folk four-in-a-row

Four-in-a-row games are much older than the plastic grid. Line-making games are one of the oldest categories of board game there is, and versions played with counters on a grid, or with pencil on paper, circulated long before anyone sold one.

The most repeated origin story is that Captain James Cook played a vertical four-in-a-row game in his cabin so obsessively that his crew called it the Captain's Mistress. It is a good story, it is attached to the game in countless product descriptions, and there is no contemporary evidence for it. Treat it as folklore — which is itself a real part of the game's history, since the name stuck.

1974: Milton Bradley makes it a standard

The game everybody knows was published by Milton Bradley in 1974 as Connect Four, with the now-iconic blue vertical grid, a sliding bar at the bottom to release the discs, and a jingle that a generation can still sing. Howard Wexler and Ned Strongin are credited with the design.

The commercial choices made that version last:

• A fixed size. 7×6 is small enough to finish in five minutes and large enough to be genuinely deep.
• Self-demonstrating rules. The physical grid teaches the gravity rule without a rulebook.
• Instant reset. Pull the bar, the discs fall out, play again — which turns one game into ten.

Solved in 1988 — twice, independently

Connect Four was solved in 1988, and remarkably it was solved twice within a month by two people who did not know about each other's work: James Dow Allen in October 1988, and Victor Allis, whose master's thesis at Vrije Universiteit Amsterdam appeared the same month.

The result: the first player wins — with the single condition that they must start in the centre column. Starting in a column adjacent to the centre loses with perfect play; the other four openings draw. The win takes until move 41 of 42 with best play on both sides.

Allis and the knowledge-based approach

What made Allis's work interesting was how he solved it. Rather than searching everything, he identified a set of strategic rules — with names like claimeven, baseinverse, aftereven and lowinverse — that describe when a player can guarantee control of a particular set of squares. Combined, these rules proved the win without exhaustive search, and they remain a standard example of knowledge-based problem solving in artificial intelligence.

The practical consequence for players is the odd–even threat theory below, which is the human-usable version of the same insight.

Connect Four as a teaching problem

Connect Four now has a second life as a programming exercise. It is the standard first project for teaching game-tree search: the rules are trivial to implement, the state fits in a 64-bit integer with the right encoding, and the game is small enough to solve on a laptop yet large enough that naive search fails.

Anyone who has written an alpha-beta search with a transposition table has almost certainly written it for this game — including the engine on this site, which deepens its search move by move until its time runs out and plays perfectly once the board is small enough to solve outright.

Common questions