In many parts of the world, gomoku has a simple goal: make five in a row. In Vietnam, caro is usually played with an extra condition that changes the character of the game. It is called chặn hai đầu, which means "blocked at both ends", and under it a five does not always win. This article looks at the custom from a player's point of view: what it says, why people like it, and how it changes the way you attack and defend.
What the custom says
Under chặn hai đầu, a line of five wins only if at least one of its ends is not blocked by the opponent. If the opponent has already placed marks at both ends of the line, the five is "dead" and does not count. The game continues.
For example, if O has marks on both sides of a row where X completes five, X has not won. If only one side is blocked, X wins.
Why players like it
A common argument for the custom is that freestyle favours the attacker too much, and there is something to it. In freestyle, the first player has a considerable advantage, and a determined attacker can often force a win. The blocked ends custom gives the defender more power: every mark placed at one end of an enemy line permanently weakens it, and a second mark at the other end kills it completely.
The result is a game with longer fights, more careful attacks and more chances for the defender to turn things around. For players who enjoy thinking, that is often more interesting than games decided in a handful of moves.
How attack changes
Under the custom, an attacker cannot simply build a long line and assume it will win. They have to ask, for every threat: if this becomes five, will it have an open end? A four sealed between two enemy marks is not a threat at all, and the defender may ignore it.
That pushes attackers toward the centre, where lines have room, and toward threats that the defender cannot seal in time. Double threats remain the main winning method, but they have to be built with open ends in mind.
How defence changes
For the defender, the custom rewards thinking one step ahead. When blocking a line, ask whether you can later block its other end too. Once both ends are sealed, that line is finished. Good defenders under chặn hai đầu keep a mental list of enemy lines that are half-sealed, and finish them when a free move appears.
It also means that some lines that look dangerous are not. A long enemy line squeezed between two of your marks does not need attention, and you can spend your move elsewhere.
The edge question
On paper, the grid often has no edge, so the question of whether the edge of the paper counts as a block rarely comes up. On a fixed board, it does. Different groups settle it differently. On this site the answer is fixed: under the blocked ends rule, the board edge never counts as a block. Only the opponent's stones can block an end. A five that runs into the edge wins unless the opponent has closed its other end.
A small example in words
Imagine X has four marks in a row on a horizontal line, with an O mark already sitting at the left end. Under freestyle, O must block the right end immediately, or X wins. Under chặn hai đầu, the situation is the same: if X completes five toward the right and the right end is still open, the five has one open end and wins. So O still blocks the right end.
Now change the picture: suppose O already has marks at both ends of the space where X would complete five. Under freestyle, X completing five still wins, so O must worry. Under chặn hai đầu, that five would be sealed at both ends and would not count, so O can ignore the line entirely and play elsewhere. The same position demands a block in one rule set and nothing at all in the other.
Agreeing before you play
Because caro is played informally in so many places, small variations exist, and part of the ritual is agreeing on the rules before starting. The most important question is simply: "chặn hai đầu or not?" Once that is settled, everything else follows. Online, the rule is set in the game and applied automatically, so there is nothing to argue about.
Playing with the custom on this site
On the home page you can play the computer with the blocked ends rule or with freestyle, and in the lobby of the online page you choose the rule for games against people. If you are used to freestyle, try a few games against the computer under blocked ends at a lower level first. The first time a five fails to win, it feels strange; after a few games it starts to feel natural.
Learning the custom as a newcomer
If you are learning caro to play with Vietnamese friends, the custom is worth practising deliberately:
- Before each attack, check that your five would have an open end.
- Before each block, ask whether it seals the line for good.
- Keep track of enemy lines that are half-sealed, and finish them off when you can.
Within a few sessions you will play the custom as naturally as the people who grew up with it.
A custom that rewards patience
The deeper effect of chặn hai đầu is on temperament. Freestyle rewards bold, fast attacks. The blocked ends custom rewards patience: the player who seals lines carefully and waits for the right moment often beats the player who rushes. That may be one reason the custom is loved by players who enjoy long, thoughtful games over a coffee.
Frequently asked questions
What does chặn hai đầu mean?
"Blocked at both ends". A five does not win if both of its ends are blocked by the opponent.
Does a five with one blocked end win?
Yes. Only a five blocked at both ends is disqualified.
Does the board edge count as a block on this site?
No. Under the blocked ends rule here, only the opponent's stones block an end; the edge never does.
Is the custom used everywhere in Vietnam?
It is very common, but caro is informal and rules vary between groups. Agree before you play.
Is the blocked ends rule harder than freestyle?
It is different. Defence is stronger, so games are often longer and attacks need more care.
