If you learned gomoku outside Vietnam and then sit down to a game of caro with Vietnamese friends, one rule will surprise you. You build a beautiful line of five, announce your win, and your opponent points at the two marks sitting at either end: chặn hai đầu — "blocked at both ends". Your five does not count, and the game goes on.
This article explains the blocked-ends rule precisely, why it exists, and how it changes the way strong players attack and defend. On this site you can choose it before any game against the computer; it is the rule most Vietnamese players grew up with.
The rule in one sentence
A line of five wins only if at least one of its two ends is not occupied by the opponent.
The "ends" are the two points immediately beyond the line, in the same direction. For a horizontal five from f8 to j8, the ends are e8 and k8. For a diagonal five from d4 to h8, the ends are c3 and i9.
If both of those points hold the opponent's marks, the five is "dead": it does not win, it stays on the board, and play continues. If one or both ends are empty, the five wins.
What counts as "blocked"?
Only the opponent's marks block. An empty point is open. Your own mark beyond the end simply extends your line to six, which is a separate question covered in overlines and exactly five.
What about the edge of the board? This is where house rules differ, because paper games often have no edge at all. Some players treat the edge like an opponent mark, others treat it as open. The safe habit is simple: before a serious game on a fixed board, agree with your opponent how the edge counts. In practice it rarely matters, because strong play happens near the centre, far from the edges.
Why the rule exists
Pure five-in-a-row strongly favours the first player. Given enough space, the first player can steadily accumulate marks, and a patient defender who only blocks one end at a time eventually runs out of answers. The blocked-ends rule gives the defender something real to aim for: if you can get a mark onto both ends of a developing line, you have not just delayed it — you have killed it for good.
That shifts the balance between attack and defence. Attackers must create threats with room to breathe, and defenders have a genuine technique beyond "block the nearest end".
How it changes defence
In freestyle, blocking one end of an open three merely turns it into a closed three; the attacker can still extend it into a closed four and force you to block again. Under blocked ends, that same one-sided block has a delayed effect: whenever the line eventually reaches five, it will be touching your mark on one side, so you only need one more mark on the other side to make the whole line harmless.
This gives defenders a habit worth learning: when you have to block, prefer the end that also blocks something else, and keep track of which enemy lines already have one of your marks at an end. Those lines are "half dead" — a single well-timed move can finish them off.
In this position the four g8–j8 is threatening five at k8, so O must play k8 anyway. The point of the diagram is what happens afterwards: the four X marks on row 8 now sit between two O marks, so they can never be part of a winning five, and O can forget about them.
How it changes attack
Attackers adapt in two ways.
First, they value open space. A line that runs toward an existing enemy mark is worth less, because it is already half blocked. Good caro players build in the direction of empty space and avoid growing lines into walls.
Second, they rely more on double threats. Since the defender can kill lines with two marks, slow single-line attacks rarely succeed. What wins in blocked-ends caro is the same thing that wins in every version of the game — a move that creates two threats at once, such as a four-three — but the defender's extra resource means you have to set it up more carefully.
A common trap: the "almost dead" four
Here is a mistake almost every player makes when they first switch to blocked ends. X has a four that is blocked at one end. The only way to finish it is at the open end. The defender must block that open end — and when they do, the line becomes dead. So a one-sided four is not a winning weapon on its own; it is a forcing move. It forces the opponent to play one specific point, which is useful for gaining time, but it does not win unless it is combined with another threat.
Beginners often "cash in" their one-sided fours too early, forcing the block and killing their own line for nothing. Strong players hold those fours in reserve and use them only when the forced reply helps them — for example, when the defender's forced mark lands somewhere useless while the attacker's four-mark builds a second line.
Does the rule affect sixes?
Under blocked-ends rules as usually played in Vietnam, the question of overlines is a separate one, discussed in the overline article. What the blocked-ends rule checks is only the two ends of the winning line. If you prefer a single simple rule, freestyle mode ignores blocking entirely: five or more in a row wins.
Practising the rule
The best way to absorb the rule is to play the computer in blocked-ends mode on the home page and watch your own fives. Every time the computer kills one of your lines with a second end mark, take back the moves with the undo button and ask: could I have extended the line in the other direction earlier, while that end was still empty?
Then do the reverse. When you are defending, count the ends of every enemy line of three or more. The lines that already touch one of your marks need only one more mark to die; the fully open ones are the real danger.
Frequently asked questions
Does a five with one blocked end win?
Yes. Under the blocked-ends rule a five wins as long as both ends are not held by the opponent.
Is the blocked-ends rule used in international tournaments?
International gomoku and renju tournaments use their own rules, which differ from Vietnamese caro. The blocked-ends rule is the everyday rule in Vietnam.
Does the edge of the board count as a block?
It depends on the house rule. Paper caro often has no edge; on a fixed board, agree on it before you start. Games are rarely decided at the edge.
Can I switch rules in the middle of a game?
No. You choose the rule set before the game starts, and it applies until the end.
Which rule makes the game more balanced?
Blocked ends gives the defender more resources, so games tend to be more balanced than pure freestyle, although the first player still keeps an advantage.
