Four in a Row Strategy: Threats and Zugzwang

Start in the middle

More lines of four pass through the centre column than through any other, and a disc there takes part in horizontal, vertical and both diagonal lines. The first player should open in the centre; the second player should answer in the centre too. Edge columns are the weakest and are usually played only when forced.

Threats and double threats

A threat is a square where, if you could play there, you would complete four. A threat on the bottom row or directly above a filled square is immediate and must be blocked. A threat higher up is a promise for later: when the squares beneath it fill, someone will be forced to play under it or into it.

Two threats at once win immediately if both are playable, because the opponent can block only one. Look for moves that create two lines of three with open ends, typically a horizontal three with both ends open on the bottom row, or a horizontal and a diagonal that meet.

Never play under a threat

If your opponent has a threat on a square, the square directly beneath it is poison: playing there lets them drop into the threat and win. Equally, if you have a threat, you want your opponent to be forced to play beneath it. The whole middle game is about who runs out of safe columns first.

Odd and even rows: the heart of the game

Count rows from the bottom, 1 to 6. In a game where both players avoid losing, the columns fill up in pairs, and the first player (Red) tends to get the odd-row squares while the second player (Yellow) gets the even-row squares. So:

• Red wants threats on odd rows (3 or 5), especially a threat on row 3 that nothing else interferes with. In the end, Yellow will be forced to play beneath it.
• Yellow wants threats on even rows (2, 4 or 6), and wants to neutralise Red's odd threats by making threats of their own in the same column or lower.

When you see a line of three with an empty fourth square that cannot be filled yet, check which row the empty square is on and which player it belongs to.

Zugzwang

The word is borrowed from chess: a position where having to move is a disadvantage. In the disc-drop game it is everything at the end. When all the safe columns are full, somebody must play under a threat. Plan for that moment from the middle game: count the safe squares left and work out who will be forced first. Against the Hard robot, this count is the only way to win, because it does not blunder tactically.

Playing second

The first player can force a win with perfect play from the centre, but perfect play is very precise and the robots at Easy and Normal do not manage it. As Yellow, answer in the centre, build even-row threats, and take any double-threat chance Red leaves. Against Hard as second player, a draw is a fine result and a win is a real achievement.

Common mistakes

1. Opening on the edge. It concedes the centre.
2. Playing under an enemy threat. It loses at once.
3. Blocking a threat that was not playable yet when a better move existed. Count first.
4. Making a threat on the wrong row. Red on even rows and Yellow on odd rows rarely cash in.
5. Ignoring diagonals. Most beginners' losses come from an unseen diagonal four.

Glossary

• Threat: a square that would complete a line of four for one player.
• Playable square: the lowest empty square of a column.
• Odd and even threats: threats on odd or even rows, counted from the bottom; they favour the first and second player respectively.
• Zugzwang: being forced to make a losing move because every safe move is gone.