Connect Four

Four in a row · 7 × 6

Red to move

0Red

0Yellow

Wins this session · 0 draws

Opponent

Show threats · squares that complete four Coach · grades your moves and explains why

Rules

Players take turns dropping one disc into any column that isn't full. The disc falls to the lowest empty space. Red moves first.
The first player to line up four of their discs in a row — horizontally, vertically or diagonally — wins.
If all 42 spaces fill up with no four in a row, the game is a draw.
Click a column or press 1–7 to drop a disc. Arrow keys move between columns.

Strategy tips

Play the centre. The middle column is part of more lines of four than any other. With perfect play, Red wins only by starting there.
Don't play under a threat. If your opponent needs one square to win, never fill the square just below it — you'd hand them the win.
Double threats win. Two winning squares at once can't both be blocked. So can two threats stacked one above the other in the same column.
Odd and even rows. As the board fills, columns run out in a fixed order. Red's threats on odd rows (1, 3, 5) and Yellow's threats on even rows (2, 4, 6) tend to come true at the end.

How the computer plays

Look-ahead search. Alpha-beta search with a transposition table, deepening move by move until its time runs out. Late in the game it searches to the end and plays perfectly.
Safety first. It always takes a win, always blocks yours, and never drops a disc under your threat.
Move ordering. Tries the centre first, and moves that create new threats before quiet ones.
Lines of four. Scores each of the 69 possible lines of four that are still open, weighting twos and threes.
Threat parity. Rates Red's odd-row threats and Yellow's even-row threats higher, because those decide endgames.
Levels. Easy spots wins and blocks most of the time but otherwise drops loosely. Medium looks 5 moves ahead. Hard thinks for about a second and a half.