Some of the most dangerous shapes in gomoku are the ones that do not look dangerous. The broken three — three marks on a line with one gap, like X X _ X — is the classic example. To the untrained eye it looks like "two and one". To an experienced player it looks exactly like an open three, because it can become an open four in one move.
This article shows how to recognise broken threes, how to defend against them, and how to use them yourself.
What a broken three is
A broken three is three of your marks on a line, within a span of four points, with one empty point between them and open space at both ends. There are two arrangements:
- X X _ X
- X _ X X
These are really the same shape seen from opposite directions.
If X fills the gap at h8, X has f8–g8–h8–i8, a solid four with empty ends at e8 and j8: an open four, which wins. So this broken three is just as urgent as a solid open three.
Why beginners miss them
The eye groups adjacent marks together. A gap breaks the group, so the brain registers "a pair" and "a single" rather than "a three". Broken threes on diagonals are the hardest of all to see, because diagonals are already harder to follow.
The fix is a counting habit: whenever you look along a line, look one point past any gap. If there is another mark of the same player just beyond a single gap, count it as part of the line.
The three ways to defend
A broken three can be defended in three places: either end, or the gap. Each has consequences.
Blocking an end. Say O blocks at e8. Now X can still fill the gap at h8 to make f8–i8, a four blocked at e8 — a simple four threatening j8. O must answer j8. The threat is contained, but X gained a forcing move.
Blocking the other end. Similar: O blocks at j8; X fills h8 to make a four threatening e8.
Blocking the gap. O plays h8. Now X's marks are split: f8–g8 on one side, i8 alone on the other. There is no way to make four on this line without extensive rebuilding. The shape is destroyed.
That is why the gap is usually the best defence: it removes the attacker's forcing move entirely, while an end block leaves the attacker a four to use later in a combination.
When not to block the gap
Occasionally an end block is better:
- If one end also sits on another dangerous line of the attacker, blocking there cuts two threats at once.
- If the gap point is useless for your own shape while an end point builds a strong line for you.
- If the attacker's potential four would be harmless because its forced reply lands on a point that helps you.
These are judgement calls. As a default, block the gap, and deviate only when you can see a concrete reason.
Broken threes in attack
For the attacker, broken threes have two advantages over solid ones:
- They are harder to see. Many opponents will overlook them, especially on diagonals.
- They are flexible. A broken three can become an open four by filling the gap, or a different four by extending an end, depending on what else is happening on the board.
A very effective attacking idea is to create a broken three on a line that crosses another of your threats. If the opponent blocks the obvious threat and misses the broken three, you win immediately. If they block the broken three in the gap, the gap point may have been exactly where they wanted to play for something else — you have forced them away from it.
Broken shapes of other sizes
The same idea applies to other lengths:
- Broken two: X _ X. It can become a broken three or a solid three.
- Broken four: X X _ X X, or X _ X X X. It is a simple four: filling the gap makes five, so it must be blocked in the gap. See open four and four.
In every case, the rule for reading is the same: look one point past each gap.
Under blocked ends
Under Vietnamese caro rules, the ends of the eventual five matter. For a broken three like f8 g8 _ i8, if O has already blocked one end (say e8), then filling the gap gives a four f8–i8 blocked at e8, threatening j8 — and completing j8 would give a five from f8 to j8 whose left end is O's e8 and right end is k8. If k8 is empty, it wins. If O manages to place a mark on k8 as well, the whole stretch is dead. Blocked-ends defenders therefore watch both the gap and the far ends of broken shapes.
Broken threes on diagonals
Everything above applies to diagonals, but diagonals deserve special mention because they combine two difficulties: the eye already struggles to follow them, and the gap makes them even harder to see. A broken three on a diagonal — say marks on e5, f6 and h8 with the gap at g7 — is one of the most common ways games are lost between intermediate players.
When you scan the diagonals through a new opponent mark, do it slowly enough to notice gaps. Count point by point: one, two, gap, three. It takes an extra half second, and it prevents the single most frustrating kind of loss in gomoku: the one where the winning line was on the board for several moves and you simply did not see it.
A quick exercise
On squared paper, draw ten short lines with random patterns of X, O and empty points. For each, answer: is there a broken three? If so, where is the gap, and what happens if you block the gap versus an end? Five minutes of this a day trains your eye quickly.
Then play the computer on the home board and try to win at least once with a broken three that the computer failed to block. If you sign in to your account, replay the game and look at how the shape was built.
Frequently asked questions
What is a broken three in gomoku?
Three marks on a line with one gap, such as X X _ X, with open space at both ends. Filling the gap makes an open four.
Is a broken three as dangerous as a solid three?
Yes, if both ends are open. It becomes an open four in one move, just like a solid open three.
Where should I block a broken three?
Usually in the gap, which splits the shape completely. Block an end only if it also cuts another dangerous line.
Why are broken threes hard to see?
Because the gap breaks the visual line, so the eye reads it as two separate groups.
