五子棋 · Cờ Caro

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How to Play Cờ Caro

Caro — gomoku, five in a row, 五子棋 — is the game people play on the back page of a notebook. You can learn it in one line: take turns marking empty squares, and the first to line up five wins. The rest of this page is about how to actually win.

The shapes that decide games

How to think about it

Every game of caro is a race between two threats, and the whole skill is knowing which threats force a reply and which do not.

• A four (four in a row with a gap you could fill) must be answered immediately, or your opponent plays five next move.
• An open three must also be answered — not because it wins now, but because it becomes a four that is open at both ends, and you cannot block two ends with one mark.
• A three that is already blocked at one end is not urgent. New players lose whole games defending shapes that were never going anywhere.
• You win by making two threats at once — the double three in the last diagram. Everything else is preparation for that.

And the flip side, which is the single most useful habit to build: before every move, look at what your opponent would build if they played where you're about to play. That square is often worth more to them than it is to you.

The three rules you can pick from

All three agree that five in a row wins; they differ on which fives count.

• Caro (chặn 2 đầu) — the Vietnamese rule, and the default here. A line of five does not win if the opponent has sealed both ends of it. The edge of the board seals a line too, since a line against the wall can no longer grow. This is the rule that keeps the game from being a first-player walkover: it gives the defender a second way to answer a five, and it makes every line you build worth checking twice.
• Standard gomoku — exactly five wins. Six or more in a row (an overline) is not a win, so you cannot fall into five by accident while making a long line.
• Free-style — five or more wins, boxed in or not. The simplest rule and the friendliest to whoever moves first.

Whichever you pick, X moves first — and moving first is a genuine advantage in this game, which is why the online ladder swaps the marks after every match.

The computer opponent

Worth being straight about what it is. It reads every empty square along all four directions and asks what a mark there would be one move away from — a five, a four, an open three — and it values a square by what it would build for it plus what it would have built for you. That is what makes it attack and defend with the same calculation. At Club it also looks three moves ahead down the most likely line, which is enough to spot a fork before walking into it.

What it is not is a solver. Beginner misses threats often and plays loosely; Casual answers fours and threes but can be out-planned; Club will punish a careless open three and is a real game for most people. None of them will out-calculate a serious caro player, and they are not meant to.

Playing online

Host a room and share the 4-digit PIN, or tap the ET room. The front two in the queue play; everyone else watches the live board. Winner stays on, loser goes to the back of the line, and the marks swap each game so the first move gets shared out. Everyone plays the host's board size and rule.