Four in a Row — the free connect-four game against a computer that improves

Seven columns, six rows, and one rule: drop a disc into a column, it falls to the lowest free slot, and the first player to line up four in any direction wins. You go first, in red. It answers in yellow.

What turns a single board game into a score chase is the opponent. Every game you win, it searches one move deeper — it starts out looking a couple of moves ahead and ends up seeing further than you comfortably can. Your score is your win streak: one loss or one draw ends the run. Free, no download, and it plays fine one-handed on a phone.

How to play

• Tap any column to drop your red disc into it. Discs stack from the bottom.
• Four in a row wins — horizontal, vertical or either diagonal.
• Your score is consecutive wins. A loss or a draw ends the run.
• The computer's search depth rises with your streak, so game five is a completely different opponent from game one.

Why the centre column is worth more than any other move

Count the four-in-a-rows that can physically exist on a 7×6 board: there are 69 of them. Now count how many pass through each column. The centre column sits inside 23 of them; the columns at either edge sit inside just 12. A disc in the middle is doing nearly twice the work of a disc on the outside, before either of you has threatened anything. Every strong engine encodes this the same way — score the centre highest, and the edges lowest — and it is the reason the first player wins this game with perfect play by taking the middle.

The subtler idea, and the one that decides most human games, is odd and even threats. Because discs stack, a threat is not "I can play here" but "this square will eventually be reachable", and whose turn it is when that square opens up is fixed by how many discs sit below it. On a board where you moved first, the squares on odd rows tend to fall to you and even rows to your opponent. That is why an experienced player will build a threat two rows above a filled column and then simply refuse to play there — waiting, because the parity of the column guarantees the opponent must open it.

Practical consequences you can use immediately. First, don't stack your own discs directly on top of each other hoping for a vertical four; it is the easiest threat in the game to block and it hands your opponent a free move each time. Second, watch for the double threat — a single move that creates two separate three-in-a-rows. That is the only thing that reliably beats a searching opponent, because it can block one end or the other but never both. And third, when the computer plays a move that looks pointless, assume it is filling a square that would otherwise have become yours. It usually is.

More games

More thinking games: Tic Tac Toe (the same idea, three in a row), Dots and Boxes, Sudoku, Minesweeper, Block Puzzle, or the other board games with a real opponent, Reversi and Checkers. Or browse all games.