The rules of gomoku fit in a sentence, but a few practical details make the game much easier to talk about: how big the board is, what happens at the edge, and how to name the points so you can write games down and follow diagrams. This article covers all three.
Why 15 by 15?
Gomoku has been played on many board sizes. Traditionally it was played on a Go board, which is 19×19. Over time, organised gomoku and renju settled on 15×15, and that is the board you play on here.
There are two reasons.
First, it is big enough. Attacks in gomoku are local. Most games are decided in a region perhaps eight or nine points wide around the first moves. A 15×15 board leaves plenty of room for the edges to stay out of the way in the opening and middle game.
Second, it is small enough. A smaller board keeps both players' marks close together, which keeps the game tactical. It also makes positions easier to show and discuss.
What about 19×19 and paper?
On a 19×19 board the game plays almost identically in the centre; the extra space mostly matters in very long games. Some players prefer it simply because they learned on a Go board.
On paper, especially in Vietnam where caro is often played on squared notebooks, the board can be effectively unlimited — players just keep drawing further along the page. This means edge effects almost never appear, which is one reason house rules about the edge vary so much.
Points and cells
On a Go board, marks go on the intersections of lines. On squared paper, they go inside the cells. The two are equivalent: a 15×15 board has 225 places to play either way. On this site the board is drawn as cells, like squared paper, because that is how most people first meet the game.
Naming the points
Every point on the board has a name made of a letter and a number:
- Columns are letters from a (left) to o (right).
- Rows are numbers from 1 (bottom) to 15 (top).
So the bottom-left corner is a1, the top-right corner is o15, and the centre is h8.
This is exactly the same scheme used for chess and many other board games, just with more columns. Once you have read a few diagrams it becomes second nature.
Common mistakes when reading coordinates
- Counting rows from the top. Row 1 is at the bottom. It helps to glance at the numbers down the left edge of any diagram.
- Skipping a letter. Unlike Go notation, which often skips the letter "i", gomoku coordinates here use every letter from a to o, including i.
- Mixing up a letter-number pair. The letter always comes first: h8, not 8h.
Reading a game record
A game record is simply a list of moves in order. Since X always starts, the first move belongs to X, the second to O, and so on. A short opening might be written:
1. h8 2. i9 3. i7 4. g9 5. j8
Odd-numbered moves are X, even-numbered moves are O. To replay a record, place the marks one by one on an empty board. You can check your understanding by setting up the position on paper and comparing it with the diagram below.
Directions on the board
When players talk about lines, they often use four directions:
- horizontal (along a row, like f8–j8),
- vertical (along a column, like h4–h8),
- main diagonal (bottom-left to top-right, like d4–h8),
- anti-diagonal (top-left to bottom-right, like d12–h8).
Every point sits on exactly four lines, one in each direction. Much of gomoku skill comes down to scanning all four directions around every new mark — yours and your opponent's.
The edge of the board
Near the edge, lines are shorter. A point in the corner lies on lines that can only extend in one direction, which makes corners poor places to build. That is why almost all games begin in the centre and why good players steer fights away from the edges when they are attacking.
Why the centre is so valuable
It is worth spelling out why every good player starts near h8. A mark on h8 lies on four lines, and each line has room for five in a row in many different positions. Count them: along row 8 alone, h8 can be part of five different five-mark windows (d8–h8, e8–i8, f8–j8, g8–k8, h8–l8). The same is true in each of the other three directions, so a centre mark belongs to twenty potential fives.
Now compare a corner mark on a1. Along row 1 it belongs to only one window (a1–e1). Along column a, only one. Along the diagonal, only one. The anti-diagonal does not even exist from a corner. A corner mark belongs to three potential fives instead of twenty. That difference is the whole reason games start in the middle.
The same arithmetic explains why marks near the edge are weaker throughout the game. Whenever you have a choice between two similar moves, prefer the one with more open space around it.
Talking about positions
Coordinates also make it easy to discuss ideas with friends or to follow articles like this one. A few conventions help:
- When describing a line, give its two end marks: "the three from g7 to i9" means g7, h8 and i9 on the diagonal.
- When describing a threat, name the point that completes it: "X threatens five at k8".
- When describing a defence, name both the move and what it blocks: "O plays e8, blocking the left end of the row".
After a week of using coordinates you will find that you "see" h8 or k11 on an empty board without counting, just as chess players see e4 or f7.
Saving and replaying games
If you sign in to your account, your games are saved with their moves, so you can replay them later one move at a time. Replaying your own games is one of the fastest ways to improve, because you see exactly where a threat appeared that you missed.
Frequently asked questions
What size is a gomoku board?
Most organised gomoku and renju use 15×15. Traditional play often used a 19×19 Go board, and paper games may have no fixed edge.
How are gomoku points named?
With a column letter from a to o and a row number from 1 to 15, counted from the bottom. The centre is h8.
Where should the first move go?
Almost always in or near the centre, because a mark there has room to grow in every direction.
Is caro played on a bigger board?
On paper it is often unlimited. On a fixed board, 15×15 is the most common size.
