Connect Four

Strategy

The rules, precisely

The board is seven columns wide and six rows tall — 42 squares. Players alternate, red first, and a disc dropped into a column always lands on the lowest free square in it, so you choose the column but not the row. The first player to line up four of their own discs wins, in any direction: across a row, up a column, or along either diagonal. A column with six discs is full and cannot be played. If all 42 squares fill with no line of four, the game is drawn. That is the entire rulebook, which is exactly why the game has lasted since 1974.

Play the centre — it is not a preference, it is arithmetic

Every square on the board belongs to some number of the 69 possible winning lines, and they are not equal. A corner square is part of three; the middle square of the bottom row is part of seven. The centre column as a whole touches more lines than any other, which means a disc there is doing more work than the same disc played anywhere else. This is why the computer opens in the centre and why strong players fight for that column early. If you take nothing else from this page: stop opening on the edges.

Look one square up before you drop

The single most common way to lose is to hand the game away. When you drop a disc, the landing square in that column moves up by one — and if the square you just created completes four for your opponent, they simply play it. Before every move, look at the square directly above where your disc will land and ask what your opponent would do with it. Experienced players spend more time on this question than on their own threats, and it is the check the computer never forgets.

The double threat wins games

A single threat — three in a row with one open end — is easy to answer: your opponent blocks and play continues. A double threat, where one move creates two separate squares that would each complete four, cannot be blocked, because they can only cover one. Almost every won game of Connect Four ends in a fork like this. Building one usually means quietly arranging discs so that a single drop opens two lines at once, rather than announcing a threat early and letting it be neutralised.

Odd and even rows: the idea behind the endgame

Here is the piece most casual players never meet. Because discs stack, a threat on an odd row (counting from the bottom: rows 1, 3, 5) and one on an even row behave differently as the board fills. With red moving first, red tends to benefit from threats on odd rows and yellow from threats on even rows — the parity decides who is forced to fill the square below the other's winning square. Strong play often comes down to steering the game towards threats of the right parity and avoiding moves that let your opponent's parity win. You do not need the theory to enjoy the game, but it explains why an apparently equal position can already be lost.

Is Connect Four solved?

Yes. James Allen and Victor Allis solved it independently in 1988: with perfect play by both sides, the first player wins — on move 41 of 42, if the opponent defends perfectly — and only by starting in the centre column. Starting in either column next to the centre draws with perfect play, and starting in the two outer columns loses. There are 4,531,985,219,092 legal positions, so a perfect opponent means a database rather than a search. The Expert level here is not perfect: it searches seven moves ahead, which is enough to punish real mistakes but not enough to force the theoretical win from move one.

What the difficulty levels actually do

Nothing is faked — there is no "computer gets extra discs" or hidden randomness beyond the easy level. Each level is the same alpha-beta search, run to a different depth. Easy looks one move ahead and plays a deliberately random move about half the time, so it misses blocks and is a fair opponent for a child. Medium looks three moves ahead: it always takes a win and blocks an immediate loss, but you can out-plan it. Hard looks five ahead and will spot a fork you are building. Expert looks seven ahead, keeps the centre, and will take any threat you leave unattended. The search is also time-capped, so on a slow phone it plays a shallower move rather than freezing the page.

Two players on one device

Switch the opponent to 2 players and the game passes back and forth on the same screen — red, then yellow, no accounts and nothing to install. It is the mode for a long train journey, a classroom, or handing a phone across a table. Undo works the same way, so you can rewind an argument about whose turn it was. The scoreboard follows red in this mode.

Playing without a mouse

The whole game is playable from the keyboard: 1 to 7 drop a disc into that column, the left and right arrows move between columns, U undoes and N starts a new game. Each column is a real button with a label a screen reader can announce, including how many spaces are left in it, and the turn indicator is a live region, so the state of the game is spoken rather than only shown.

Variants worth knowing

The standard game is 7×6, but the family is larger. Pop Out lets you remove one of your own discs from the bottom of a column instead of dropping a new one, which makes the board shrink and grow. Five in a Row raises the target on a bigger grid. Power Up adds special discs. And the same core idea — gravity plus a line of four — turns up in dozens of physical sets under different names. If you enjoy this, Tic Tac Toe is the same genus with a smaller board, and Sudoku or Minesweeper scratch the same itch on your own.

Nothing here leaves your device

The game runs entirely in your browser. There is no account, no server keeping your games, and no move is transmitted anywhere — the opponent's search happens on your own machine, which is also why it answers instantly. The scoreboard and your settings are kept in your browser's local storage; clearing your browser data resets them, and nothing is stored anywhere else.