Connect Four

Connect Four is a two-player game on a seven-by-six grid stood on its end. You drop a disc into a column, it falls to the lowest free square, and the first player to line up four — horizontally, vertically or diagonally — wins.

Play against a bot at two strengths, or pass the device back and forth with someone else.

How to play

1. Click any column that is not full to drop your disc.
2. Discs stack from the bottom, so you choose the column and gravity chooses the row.
3. Four in a row in any direction wins. A full board with no four is a draw.

Why the centre column matters so much

Every possible four-in-a-row on the board passes through some set of squares, and the centre column participates in more of them than any other. Counting them out: a disc in the centre column can be part of thirteen different winning lines; a disc in an outer column, only three.

That imbalance is the whole opening theory of the game. The first move in the centre is strong enough that Connect Four is a first-player win with perfect play — a result proved in 1988 by James Allen and, independently, Victor Allis.

The move above your move

The most reliable way to lose is to forget that placing a disc makes the square above it playable.

Before dropping into a column, look at what sits above the square you are about to fill. If it would complete a four for your opponent, you have just handed them the game. Strong players spend more time on this than on their own threats — most Connect Four games are decided by a gift rather than a plan.

Double threats and parity

Since any single threat gets blocked, winning means creating two at once. The classic shape is a horizontal three with open squares at both ends, which cannot be answered.

Beyond that is parity: because discs stack, a threat sitting on an odd-numbered row tends to fall to the player who moved first, and one on an even row to the second player. Setting up your threats on rows that favour you, and your opponent’s on rows that do not, is what separates a good club player from someone who has only learned to block.

Tips

• Take the centre column early. It takes part in more possible fours than any other column, and games where one side owns the centre are usually decided by it.
• Watch what your move gives away. Dropping a disc raises the square above it into play, and handing your opponent a winning square directly above yours loses more games than any other single mistake.
• Aim for a double threat — two separate places where your next disc completes a four. A single threat gets blocked; two at once cannot be.
• Count parity on the edges. Because discs stack, threats on odd rows favour whoever moved first and threats on even rows favour the second player. Steering your threats onto the right rows is what strong play looks like.