How to play Connect Four

Connect Four is played on a grid seven columns wide and six rows deep, stood upright so that discs fall. You take red, the computer takes yellow, and the first drop alternates between rounds so neither side keeps the advantage of moving first.

The move

On your turn you choose a column. The disc falls to the lowest empty slot in that column — you never choose the row, only the column. A column with six discs in it is full and cannot be played.

That single constraint is the whole game. You cannot place a disc where you want it; you can only place it on top of what is already there, which means every move you make also builds a platform for your opponent.

Winning

The winner is the first player to get four of their own discs in an unbroken line — horizontally, vertically, or along either diagonal. If all forty-two slots fill with no line of four, the game is a draw.

The threat, and why an open three is not always winning

A "threat" is an empty slot that would complete four in a row. The game is really a competition to create a threat your opponent cannot answer.

Three in a row with empty playable slots at both ends looks unanswerable, and beginners often play for it. It usually is not, because your opponent only has to block one end this move and the other next move — unless both ends are playable *now*, which is the pattern to actually aim for.

The genuinely unanswerable shape is the double threat: two separate slots that each complete a four. Your opponent gets one move and there are two holes to plug. Almost every win at a decent level is a double threat, and almost every loss is failing to see one arriving.

The trap under the threat

The subtlety that separates a good player from a beginner is the vertical stacking rule. If you have a threat sitting in a slot, and your opponent plays the slot directly beneath it, they hand you the win. So a threat is not only a way to win — it is a square your opponent is now forbidden to play beneath, which quietly removes a column from their options.

The reverse is the most common way to lose: playing a slot that puts your disc directly underneath a slot your opponent needs. Before dropping anywhere, look at what your disc will sit under.

Parity, briefly

Because the board is six rows deep and the columns fill from the bottom, threats on odd rows and even rows behave differently depending on who moves first. This is worth knowing exists even if you never count it: an experienced player creating threats on the rows that suit them is not guessing.

The three strengths

The computer runs a look-ahead search rather than a set of rules. Easy reacts a move or two out. Medium plans several moves deep. Hard searches far enough that cheap tricks never work.

At every setting it always takes an immediate win if it has one, and always blocks an immediate loss if you have one. So a win against it is a real win — it never simply overlooks the obvious.