Gomoku is one of the simplest games you will ever learn and one of the hardest to master. You can explain the whole thing in one sentence — take turns placing marks, and the first player to make five in a row wins — yet players spend years learning how to see the threats that decide a game. This article covers everything you need before your first game: the board, the turn order, what exactly counts as a win, and the two rule sets you can choose from on this site.
In Vietnam the same game is called caro, and it is probably the most widely played pencil-and-paper game in the country. Generations of students have played it on the squared pages of school notebooks. The rules below apply to both names; where the Vietnamese version differs, we say so.
The board
You play on a grid. The standard board for gomoku is 15 by 15, which gives 225 points. Some players use 19 by 19, the size of a Go board, and on paper people often play on an open-ended grid with no edge at all. On this site the board is 15×15, which is the size used in most organised play and large enough that the edges rarely matter in the opening.
Each point is named by a letter for the column and a number for the row. Columns run from a on the left to o on the right; rows run from 1 at the bottom to 15 at the top. The centre of the board is h8. This naming is only for talking about games — you never need it to play — but it makes diagrams and game records possible, and every example on this site uses it.
The pieces
There are no pieces to move. Each player has an unlimited supply of marks. On paper the marks are X and O; on this site the first player is X (shown in red) and the second player is O (shown in blue). Once a mark is placed it never moves and is never removed. That permanence is part of what makes gomoku so sharp: every move you make stays on the board for the rest of the game, for better or for worse.
Taking turns
X always moves first. Players then alternate, one mark per turn, on any empty point. There is no passing and no restriction on where you may play, apart from the special rules some tournament variants add (more on that below).
Most players start in the centre, because a mark there can grow in all eight directions. You are not required to, but you will quickly see why nearly everyone does.
Winning: five in a row
You win by making an unbroken line of five of your own marks in a row — horizontally, vertically or diagonally. The line must be continuous: five marks with a gap between them do not count, and marks of the other player in the line break it.
Diagonal fives count exactly the same as straight ones. Beginners often miss diagonal threats because the eye naturally follows rows and columns, which is one reason diagonal attacks win so many games at the beginner level.
The two rule sets on this site
Before each game against the computer you choose one of two rule sets. They agree on everything except what happens when your five is surrounded.
Blocked ends (Vietnamese caro)
In Vietnamese caro, a line of five does not win if both of its ends are blocked by the opponent. If there is an O mark directly before the first X and directly after the last X, the five is considered "dead" and the game goes on. A five with at least one open end wins normally.
This rule exists to reward attack over patient accumulation. If you can only finish a line when at least one end is free, you have to create threats your opponent cannot fully contain, rather than slowly building a line inside a wall of enemy marks. It changes defensive technique too: in blocked-ends play, closing one end of a line early is often enough to neutralise it later.
Freestyle
In freestyle gomoku, five or more in a row wins, regardless of what is at the ends. A line of six or seven also counts. This is the simplest possible rule and the one most people play casually around the world.
Neither rule is "the correct one". Freestyle is easier to learn and good for beginners; blocked ends is the everyday rule in Vietnam and rewards a more active style. We explain the differences in more depth in freestyle vs blocked ends.
What about exactly five and overlines?
Some rule sets care about overlines — lines of six or more. In "standard" gomoku as played in some tournaments, only exactly five wins and an overline does not. In freestyle on this site, six or more also wins. We cover the reasoning in overlines and exactly five.
You may also hear of renju, a professional variant that puts restrictions on the first player to balance the game. Renju forbids the first player from making certain combinations, such as two open threes with one move. Those restrictions are not used here; if you want to learn renju later, everything you learn about threats in ordinary gomoku still applies.
How a game ends
A game ends when:
- one player completes a winning five,
- one player resigns,
- or the board fills up with no winner, which is a draw. On a 15×15 board this is rare between players who attack, but it does happen.
Playing online adds two more endings: running out of time on the clock, and leaving the game. The details are in how a gomoku game ends.
Your first game against the computer
The fastest way to understand the rules is to play. Open the board on the home page, choose freestyle for your first few games, and set the computer to its easiest level. You will be X and move first. Try to build lines in two directions at once and watch how the computer responds; when it blocks one of your lines, notice where it put its mark and why.
If you make a mistake, the undo button takes back your move and the computer's reply. When you have had enough, the leave button ends the game without fuss — there is no penalty for leaving a game against the computer.
Once freestyle feels comfortable, switch to blocked ends. You will immediately feel the difference: lines you thought were winning suddenly need an open end, and you will start to value space around your stones.
Common rule misunderstandings
"Four in a row wins." No — you need five. A four is a threat, not a win, though an open four (with both ends empty) is almost always unstoppable, because the opponent can only block one end.
"A line with a gap counts." No — the five must be continuous. A pattern like X X _ X X is one move away from five, which makes it dangerous, but it is not a win yet.
"Only straight lines count." No — diagonal lines count exactly the same.
"Under blocked ends, a blocked four is harmless." Not quite. A four blocked at one end still threatens to become five at its open end, and that five would have one open end — so it wins. Only when both ends end up blocked is the line dead.
Where to go next
The rules take five minutes; seeing threats takes much longer. Once you have played a handful of games, read about open and closed threes and the open four, which are the building blocks of every attack. The beginners section collects the articles we recommend in order.
Frequently asked questions
Who moves first in gomoku?
X moves first. On this site X is shown in red. Moving first is a real advantage, which is why serious variants like renju add restrictions for the first player.
Does a diagonal five count?
Yes. Horizontal, vertical and diagonal lines of five all win equally.
What is the difference between caro and gomoku?
They are the same family of game. "Caro" is the Vietnamese name, and in Vietnam it is usually played with the blocked-ends rule. "Gomoku" usually refers to the freestyle version or to standard tournament gomoku.
What happens if the board fills up?
If every point is filled and nobody has a winning five, the game is a draw.
Which rule set should I learn first?
Freestyle, for a few games. Then switch to blocked ends if you want to play the way most Vietnamese players do.
