Connect Four

Drop your discs and line up four before your opponent.

The great classic of the 7 × 6 vertical grid. At Master level, the AI relies on a precomputed opening book and solves the game as soon as it can.

Players: 1 or 2 players
Length: 5 min

Against the AI
Two players on one device

Play Connect Four

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How to play

Goal

Two players take turns dropping a disc of their colour into one of the seven columns of a vertical grid with 6 rows. The disc falls to the lowest empty square. The first to line up four discs horizontally, vertically or diagonally wins.

How it works

The gold disc always goes first.
A full column can no longer take a disc.
If the grid fills up with no line, the game is a draw.
Click or tap a column; on the keyboard, keys 1 to 7, or arrows then Enter.

Tip: the centre column takes part in the greatest number of lines. Try to create two threats at once, and watch the parity: the first player benefits from threats on odd rows (1, 3, 5), the second from even rows.

About the AI

The engine represents the grid with bitfields and explores moves with the negamax algorithm (alpha-beta pruning, transposition table, threat look-ahead). Its evaluation takes into account the position of the discs, the threats and their parity.

At Master level, the AI first consults an opening book computed offline by an exact solver, then solves the game mathematically as soon as possible: if it has a winning position, it does not let it slip. The lower levels search less deeply and deliberately add inaccuracies.

AI levels

BeginnerLooks two moves ahead and is often caught by surprise.
AmateurSees simple threats, but lacks a plan.
IntermediateCalculates seven moves ahead: a solid opponent.
ExpertDeep search and careful reading of threats.
MasterOpening book, exact solving and deep search: nearly perfect.