Connect Four looks like tic-tac-toe with a bigger board. It isn’t. Gravity changes everything: you can only ever play the lowest empty hole in a column, so every disc you drop also decides who gets the hole above it. Half the game is making threats; the other half is making sure your opponent is the one forced to play underneath them.
The rules take thirty seconds. The ideas — double threats, tempo, and a surprisingly deep point about odd and even rows — will keep you busy for much longer.
How to play
The rack has 7 columns and 6 rows: 42 holes. Players take turns; on your turn you drop one disc into any column that still has room, and it falls to the lowest empty hole. You cannot pass, and discs never move once they land.
You win the moment you have four of your discs in a straight line: across a row, up and down a column, or along either diagonal. Lines of five or more also count. The game ends immediately, so the rack is often far from full.
If all 42 holes are filled and nobody has four in a row, the game is a draw. Each player gets exactly 21 discs.
Below, columns are numbered 1 to 7 from the left (the same numbers you can press on the keyboard), rows 1 to 6 from the bottom, X is the first player and O the second. In the diagrams, X discs are solid red and O discs are black with a pale ring, just like the rack above.
A worked example
This is the fastest trap in the game, and it is played on the bottom row.
X drops in the centre, column 4. O stacks on top of it. X drops in column 3, next to the first disc. O, not seeing the danger, stacks on column 3 as well.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 |
(Only the bottom two rows are shown; the rest are empty.)
Now X drops in column 5. The bottom row reads X X X in columns 3–5, and both ends are open: column 2 and column 6 each complete four.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 |
O can block only one end. X plays the other and wins with four discs. O’s mistake was move four: after X held columns 3 and 4 on the bottom row, O had to take column 2 or column 5 straight away.
Strategy
Win, block, and don’t play underneath
Every turn, ask three questions in order. Can I win now? Can my opponent win next move — and if so, where must I block? And will my move put a disc directly under a hole that wins for my opponent? If it will, you have just handed them that hole. The Easy computer forgets these checks half the time; that is why it is Easy.
Take the centre
A hole is only useful if it can be part of a four. Count the possible fours through each hole and the middle of the rack dominates:
There are 69 possible lines in total, and every horizontal one passes through column 4. Open in the centre and keep contesting it.
Double threats
One threat can be blocked. Two can’t. The bottom-row trap above is the simplest double threat; the most common one in real games is two of your winning holes stacked directly on top of each other in the same column. Your opponent must block the lower one — and in doing so gives you the hole above it.
Threats you can’t use yet
A winning hole floating in mid-air is not wasted. Nobody wants to play the hole beneath it: if your opponent does, you win; if you do, they block. As the rack fills, those “don’t play here” columns pile up, and eventually someone runs out of safe moves. That is the endgame of most strong games.
Odd and even rows
Here is where it gets deep. Suppose the safe moves run out and the players simply fill columns turn by turn. Because the first player moves on turns 1, 3, 5… and each column has an even number of holes, the first player tends to end up with holes in odd rows (1, 3, 5) and the second with even rows (2, 4, 6). So as the first player, a floating threat on an odd row is worth far more than one on an even row; as the second player, the reverse. Grandmaster-level Connect Four is largely the art of arranging threats on the right parity.
The idea underneath
Three ideas carry the whole game.
A threat is a hole that would complete your four. A tempo is the right to make the next free move: when you make a threat your opponent must answer, you keep the tempo, and you get to choose where the next fight happens.
Zugzwang is a German chess word for “compulsion to move”: a position where any move you make hurts you, and you’d rather pass — but you can’t. Connect Four is full of zugzwang, because every disc props up the hole above it. The player who is forced to play under their opponent’s threat loses, and the odd/even rule above is really a way of predicting who will be forced.
Connect Four is solved. In 1988, James D. Allen and Victor Allis independently showed that the first player can always win by starting in the centre column. Starting in either column next to the centre lets the second player hold a draw, and starting in any of the four outer columns loses against perfect play. Allis’s solution rests on exactly the ideas on this page: it is a catalogue of rules about threats, parity and zugzwang, backed by computer search.
The computer on this page searches instead of knowing. On Hard it uses alpha-beta search: it imagines up to a dozen moves ahead, skips any line that is already worse than one it has found, and scores unfinished positions by counting open lines of two and three. It tries central columns first because they are usually best, which lets it skip more. It is strong, but it has no solved database behind it — a patient player can find its blind spots.
Common mistakes
- Stacking on your opponent’s disc by reflex. It feels safe and often leaves a bottom-row trap wide open.
- Blocking a threat and creating one for them. Check what your blocking disc props up.
- Chasing one line. A single three-in-a-row is easy to block. Build towards two threats that share discs.
