Connect Four

How to play

The grid is seven columns wide and six rows deep, which gives forty-two squares in total. On your turn you choose a column, not a square: the disc slides down and settles on the lowest empty spot in that column. You pick the column, gravity picks the row, and that single restriction is what makes the game interesting. The first side to line up four of its own discs wins, and the line may run horizontally, vertically, or along either diagonal. If all forty-two squares fill with no line of four, the game is a draw.

One side opens each game and the New game button alternates which side that is, so the first move passes back and forth instead of always belonging to you. That matters more than it sounds: moving first is a measurable advantage on a board this size, so a single game decides as much about who opened as about who played better. Against the computer you are always red; the machine plays yellow and opens every other game.

Controls

• Mouse or touch: press the numbered button above a column, or tap anywhere inside the column on the grid itself.
• Keyboard: the digits 1 through 7 drop a disc in that column. The column buttons are also normal focusable buttons, so you can reach them with Tab and press Enter.
• Undo: steps back one full round — your move and the computer's reply — so one careless drop does not have to end the game. In two player mode it steps back a single move.
• New game: clears the board and gives the opening move to the other side. The running match score above the board keeps counting until you clear your browser storage.

Playing against the computer

The three computer levels are Casual, Sharp and Ruthless, and they differ in exactly one thing: how many moves ahead they look. Casual searches two plies, Sharp four, and Ruthless six, where a ply is one disc dropped by one side. Before any of them searches, all three run the same two checks — can I complete four right now, and can my opponent complete four right now? So even Casual will never stroll past an obvious four in either direction. What you gain by moving up a level is foresight in quiet positions, where nothing is urgent yet and the game is being decided anyway.

Underneath, the opponent is a negamax search with alpha-beta pruning rather than a bag of handwritten rules. When it reaches the end of its lookahead it grades the position by walking every group of four squares that could hold a winning line — there are sixty-nine of them on this board — and counting how many of its own discs and how many empty squares each group contains. Groups that already hold discs from both sides are worth nothing to anybody and score zero. On top of that it adds a bonus for every disc it owns in the center column. It also tries the center column first and works outward toward the edges, which sounds cosmetic but is what makes the pruning cheap enough to run inside a page.

Strategy that changes your results

Start in the center column and keep fighting for it. This is not folklore, it is arithmetic: of the sixty-nine possible four-in-a-row lines on a seven by six grid, fifty-one pass through the center column, while only fifteen pass through either outside column. Every disc you place in the middle belongs to more potential lines than a disc anywhere else, and every disc your opponent places there does the same for them. If you only change one habit today, change this one.

Next, learn to see threats instead of discs. A threat is an empty square that would complete four for somebody. A single threat is a nuisance — your opponent simply fills the square. The game is actually won by making two threats at once, because your opponent can only block one of them per turn. That is why an unblocked row of three with empty playable squares at both ends is not a threat but a victory, and why strong players build quietly toward positions with two open ends rather than racing to get three in a row anywhere.

Finally, watch what your move gives away. Because discs stack, every disc you place hands your opponent the square directly above it. Dropping into a column so that the square above completes their four is the most common way an otherwise decent player loses, and it happens most often when you are absorbed in your own vertical stack while they build across the board. Before you commit, picture your disc landing, then look at the square on top of it and ask whether you would be happy to see their color there.

Where the game came from

Lining up four marks in a grid is much older than any commercial product; people have played versions of it with pencil and paper for generations. The familiar plastic version with a standing blue grid and two trays of discs became a mass market toy in the 1970s, and the name most shoppers know is a registered trademark belonging to its publisher. Boss Games has no connection with that publisher or with any other board game company, and nothing here is licensed from anyone. The board you are playing on, the search engine behind it and every word on this page were written for this site.

The game also has a place in computing history, because it is small enough to be settled completely by machine and large enough that doing so was an achievement. That work was published in the late 1980s, and the outcome is covered in the strategy guide linked below.

Questions players ask