Gomoku rules

How this house plays, plus the international rules of gomoku and renju.

The international rules follow the official texts of the Renju International Federation (RIF), keeping only what matters at the board; clocks, referees and the like are left out.

Board and first move

A 15×15 board, 225 points. Players take turns placing one stone on an empty point; stones never move. Black goes first, and the first stone goes on the center point (H8).

Coordinates: columns A to O (I included), rows 1 to 15 counted from the bottom. Game codes such as “H8I9J10…” use exactly these.

Terms

Five: Five stones of one color unbroken in a line — across, down or diagonal.
Overline: Six or more stones of one color unbroken in a line.
Four: A line that one more stone turns into a five.
Straight four: A four with two points that make five. It cannot be stopped.
Simple four: A four with only one point that makes five, including the split four with a gap inside.
Three: A line that one more stone turns into a straight four — without making a five at the same time.
Sleeping three: A three that can only become a simple four, never a straight four. It does not count as a three and plays no part in double-threes.
Double four: One stone making two or more fours at once. Two fours on the same line count.
Double three: One stone making two or more threes at once, the stone being part of each.

Top: a straight four — both A points make five. Middle: a simple four, one end blocked by white, only A left. Bottom: a split four; the A in the gap is the only five point.

Top: a three — either A makes a straight four. Middle: a split three; filling A also makes a straight four. Bottom: a sleeping three, blocked at one end; at best a simple four, so not a three.

Winning and losing

The first to make a five in any direction wins.
White has no forbidden moves; a white overline also wins.
Black does: a black stone that makes an overline, a double four or a double three — without also making a five — loses the game.
If black's stone makes a five and a forbidden shape at once, the five counts and black wins.
Resigning or running out of time loses.

Over the board, white must point out a double three or double four before playing the next move; an overline can be claimed any time before the game ends. Online, the system rules on it the moment the stone lands.

Forbidden moves

Only black is bound. Moving first is a big edge — unrestricted, black has a forced win. Barring the three strongest shapes brings the two sides roughly level.

A: F8–G8 across and H9–H10 down each become a three, crossing at A — a forbidden double three.

A: a four across and a four down — a forbidden double four.

A double four can sit on one line: after A, both E8 and I8 make five, so there are two fours.

A: six in a row — a forbidden overline.

Four-three is allowed: A makes a four across and a three down, black's most common way to win.

A: five across and a four down at the same time — the five comes first, black wins.

Real and false threes

A double three only counts real threes (RIF rule 9.3). Whether a three is real depends on whether its straight-four points can actually be played:

if the straight-four point makes a five at the same time, the three is false;
if the straight-four point is itself an overline, a double four or a double three, the three is false;
if at least one straight-four point can be played normally, the three is real.

Only two real threes make a forbidden double three. If one is false, only one three is left, and the move is allowed.

A: the three down is real. The three across is blocked by white on the left and can only become a straight four at B — but B makes five in column I first. That three is false, so A is allowed.

Nested forbidden moves

Whether a straight-four point is forbidden can itself need another check, and so on down. The method is to fill in: put a black stone on A, then judge the straight-four point B; if B depends on a further point, fill B in as well and keep going until one level settles on its own, then work back up.

A: the three down is real. The three across is blocked on the left and can only become a straight four at B; but with A filled in, B makes a four, a three down and a three on the diagonal — itself forbidden. So the three across is false, and A is allowed.

When each level depends only on the next, a rule of thumb: an odd number of levels means A is forbidden, an even number means it is not. More than two levels are rare in real games.

White can also win through the rule: using fours to force black to defend on a forbidden point. So black has to watch which points become forbidden, and white which forbidden points are really false.

Draws

The board is full.
Both agree: one offers when making a move, the other accepts.
Both players pass in succession (no passing within the first three moves).

Tournament openings

Black's first-move edge is large, so in official play black does not open freely. The opening rule (RIF 1996):

The first three stones must form one of 26 standard openings: black 1 on the center, white 2 touching it straight (direct) or diagonally (indirect), black 3 within the central 5×5; 13 of each.
Swap after three: the tentative black places the first three stones; the tentative white may then swap colors.
White plays the 4th stone anywhere.
Two fifth moves: black offers two non-symmetrical 5th moves, white keeps one and plays the 6th, and the game goes on normally.

Example (Kagetsu): black 1 on the center, white 2 direct, black 3 within the central 5×5; after the swap white plays 4, black offers two 5s, white keeps one and plays 6.

RIF has since used other tournament opening rules as well. This house lets black open freely by default; five-in-a-row tables can choose a simpler swap opening (place, then choose).