Most gomoku games between reasonable players end the same way: one stone creates a four and an open three at the same time. The defender blocks the four, because five would win immediately, and then the three becomes an open four with two winning points. Game over. This is the four-three, and learning to survive it is mostly about seeing it one move before it happens.
Why the four-three works
A four, open or closed, must be answered at once. An open three must be answered on the next turn. When a single stone creates both, the defender has one move and two problems. Blocking the four is forced, so the three is left alone, and a free three turns into an open four.
The same logic explains the four-four (two fours at once, which is even stronger) and the three-three (two open threes at once). Under freestyle rules all of these win. Some international rule sets forbid the three-three or four-four for the first player; the rules on this site do not, so expect them.
Spotting the landing point
A four-three needs two lines that meet at an empty point: one line with three stones already in a row (to become four) and another with two stones (to become three). The meeting point is where the blow will land.
To find these points during a game, use the crossing test after every enemy move:
- Take the stone the opponent just played.
- Look along the four lines through it and count enemy stones within a stretch of five points.
- If one line has three and another has two, and they meet at an empty point, that point is a landing point.
- Also check the lines through the opponent's older stones that the new stone connects to; the crossing is not always at the newest stone.
Once found, occupying the landing point breaks both lines at once. It is the most efficient defensive move in gomoku.
When you cannot prevent it
Sometimes there are two landing points, or you are busy answering another threat, and the four-three arrives anyway. You are not automatically lost. Before blocking, check one thing: do you have a four of your own?
A four forces the opponent to answer. If you can play a four, and then another, and another, each one gains you a tempo. If one of your forcing moves happens to land on the key point of the opponent's shape, you break it without losing time. And if your chain ends in a four-three of your own, you win first.
This counter-attack only works with fours. A three does not force anything once the opponent has a four on the board.
Blocking the four: which end
If you have no counter, block the four. When the four is open at both ends, you cannot stop it anyway. When it is closed, there is only one point to block. When the four is split (with a gap, such as g8, h8, j8, k8), block inside the gap.
After that, look at the three. Sometimes the opponent's open three is not quite open: an edge of the board, or one of your stones two points away, may mean it can only become a closed four. In that case you have survived. This is why stones you placed earlier, even ones that looked useless, often save games later.
The blocked ends rule and double threats
Under the Vietnamese blocked ends rule, a five only wins if at least one end is open. This can rescue a defender in a small number of positions: if the line that the attacker's three will grow along is already closed at a distance on both sides by your stones, the resulting five might be closed at both ends and not count. It is worth checking, especially near your own stones. In freestyle there is no such rescue.
Building a defensive memory
The four-three follows a small number of shapes. After a few games you start recognising the "L" shape (a line and a short perpendicular line meeting at a corner), the "T" and the "X" crossing. A good exercise is to replay your lost games and, for each loss, find the move where the landing point first existed. Then ask what you played instead. Almost always you were blocking something less important.
A short defensive routine
Turn the ideas above into a routine you run on every move in the middle game:
- First, look for enemy fours. If there is one, block it and stop thinking about anything else.
- Second, look for enemy open threes, solid or split. Block one, choosing the end that also helps you.
- Third, run the crossing test on the opponent's last two stones. If a landing point exists, take it unless you have a forcing move.
- Fourth, if nothing is urgent, look for your own fours and threes.
Players who skip the third step lose most of their games to four-threes. Players who run it every move lose far fewer, even against stronger opponents, because the attacker has to find a new way to win and often runs out of ideas.
The routine also changes how you see the board. After a few weeks you stop looking at lines one by one and start seeing the points where lines meet. That shift is the real sign that your defence has improved.
Practise
Play the computer on the home page at the third or fourth level and try to lose as few games as possible to a four-three. Use undo to step back to the move before the blow and find the landing point. Related reading: defending against an open three and how to block in gomoku.
Frequently asked questions
What is a four-three?
A single stone that creates a four and an open three at the same time. The defender must block the four, and the three then becomes an open four.
Can I defend a four-three once it is on the board?
Only if you have fours of your own to play first, or if the three is not really open because of an edge or your stones.
What is a landing point?
The empty point where two enemy lines meet, one with three stones and one with two. A stone there would create a four-three.
Is a three-three as strong as a four-three?
It usually wins too, though it gives the defender one extra chance to answer with fours. Some international rule sets forbid it for the first player; this site does not.
Does the blocked ends rule protect me?
Occasionally. If the attacker's future five would be closed at both ends by your stones, it does not count. Check near your own stones.
