Gomoku

Rules and how to play

The board is made of 15 vertical and 15 horizontal lines, and stones are placed on the points where the lines cross (intersections). Black moves first and White moves second, and the players take turns placing 1 stone at a time. A stone that has been placed is never moved or captured.^[3][5]

The first player to get 5 of their stones in an unbroken line vertically, horizontally, or diagonally wins. Under Freestyle rules, 6 or more in a row also wins. If the board fills up and neither player has made 5 in a row, the game is a draw.^[3][5]

Five, four, straight four, and three (names of shapes)

A line of 5 stones and the shapes that come before it have the following names. The names follow the rules of the Renju International Federation (RIF), and this page also uses them to explain the Renju rules and the strategy tips.^[4]

• Five: 5 stones in an unbroken line. The first player to make a five wins.
• Four: a line that becomes a five with 1 more stone. It is also a four when your opponent has blocked one end of 4 stones in a row, or when the line has a single gap (2 and 2, or 3 and 1).
• Straight four: 4 stones in a row that become a five with a stone at either end. Your opponent cannot block both ends with 1 move, so you can make a five on your next move.
• Three: a line that becomes a straight four with 1 more stone. Besides 3 stones in a row, a line of 2 stones and 1 stone with a single gap between them is also a three if 1 more stone can make it a straight four.

Renju rules (forbidden moves for Black)

Renju rules set forbidden moves for Black only, so that Black, who moves first, does not have an advantage. Black cannot play the following 3 moves. White has no forbidden moves, and 6 or more in a row also wins for White.^[3][4][8]

• Double-three: a move that makes 2 threes at once with a single stone.
• Double-four: a move that makes 2 fours at once with a single stone. It is also a double-four when the 2 fours are on the same straight line.
• Overline: a move that makes 6 or more in a row. Black wins only by getting exactly 5 in a row.

In standard Renju, Black loses by playing a forbidden move. Under the Renju rules on this page, on Black’s turn a × appears on each intersection where a move would be forbidden, and Black cannot play there. A move that makes exactly 5 in a row can be played and wins for Black, even if it also makes a double-three, a double-four, or an overline. If the only intersections left for Black are forbidden points, this page counts the game as a draw.^[3][4][7]

When counting threes, this page follows the RIF rules and does not count a three if the stone that would make it a straight four would make a five at the same time, or if the point that would make it a straight four is a forbidden point. In standard Renju, Black’s first move is played at the center of the board, and in tournaments, opening rules decide how the first few moves are played. This page uses neither rule, and you can play the first move on any intersection you like.^[1][4]

Controls on this page

To the right of the board (below it on narrow screens), first choose the rules (Freestyle or Renju), then choose how to play. With the Easy, Medium, and Hard buttons under “Play vs computer”, you play first (Black). To play second (White), press “Play second (White) · Medium”. With “2 players on this device”, 2 people take turns playing Black and White on one device.

Press “Play a friend (send a link)” to create a room and show a link. Send the link to your opponent. When they open it and press “Join”, the game starts. The host plays first (Black) and the player who joins plays second (White), and the game uses the rules chosen before the room was created. After the game ends, press “Rematch (swap sides)” to start the next game. If your opponent stays disconnected for 2 minutes, you can press “Continue vs computer” to keep playing against the computer.

With a mouse, click an intersection to place a stone. With touch controls, such as on a phone, the first tap shows a faint stone where it will go, and tapping the same intersection again places it. Drag the board to rotate the view, and zoom in and out with the mouse wheel or 2 fingers. Switch the view with the “Angled” and “Top-down” buttons at the top right of the board.

You can also play from the “Button controls” table to the right of the board (below it on narrow screens). Row 15 is at the top of the table and column A is at the left. With a keyboard, move between intersections with the arrow keys and press the Enter key to place a stone. A ring marks the last stone placed, and the last move (for example, Black H8) appears below the table.

Against the computer and in “2 players on this device”, press “Undo move” to take back a move. Against the computer, this takes back the computer’s move and your own move together and returns to your turn. Press “Play again” to start a new game with the same rules, and press “Resign” to admit defeat and end the game. Press “Change mode” to return to the screen where you choose the rules and how to play.

Strategy tips

• When your opponent makes a four, immediately block the point that would make a five. If you leave a three alone, it can become a straight four that makes a five at either end and cannot be stopped, so block one end while it is still a three.
• Aim for a “four-three”, a single move that makes a four and a three at the same time. After your opponent blocks the four, you can turn the three into a straight four on your next move, so you win if your opponent has no move that makes a four. Under Renju rules, Black cannot play a double-three or a double-four, so making a four-three is the basic way for Black to win.
• If you make fours one after another, your opponent has to block the point that would make a five each time. After each block, look for a line where you can make the next four. You can then keep attacking while your opponent can only block, and finally win by making a four-three or a straight four.
• When you play White under Renju rules, look for a four where the point Black must block is one of Black’s forbidden points. Black cannot play there, so unless Black can make a five first, White makes a five on the next move and wins.

Origin and history of Gomoku

A picture scroll from the Heian period (794–1185) shows court ladies in twelve-layered robes (junihitoe) playing with white and black stones placed in the center of a board, and Nihon Renjusha (Japan’s renju association) presents it as a picture that appears to show Renju.^[1]

In the mid-1700s, in the middle of the Edo period (1603–1868), the Nijo family in Kyoto called this game itsutsu-ishi (五石, “five stones”). Kuwanaya Buemon (the 10th of that name), a merchant of bintsuke hair oil who visited the Nijo family, was skilled at the game, and he is said to have spread it among ordinary people.^[1][7][8]

The book “五石定蹟集” [Collection of Itsutsu-ishi Standard Openings], written in 1856 by Kuwanaya Buemon, the 13th of that name, is said to be the oldest surviving book on Renju. After that, different schools called the game by names such as itsutsu-ishi, kakugo (格五), and gomoku narabe (the full Japanese name of Gomoku), and many guidebooks were published.^[7][8]

In 1899, Kuroiwa Ruiko, who ran the newspaper the Yorozu Choho, published a winning strategy for Gomoku in the paper, invited readers to propose a name for the game, and on December 6 announced that it would be called Renju (聯珠, later written 連珠). The new name unified the various earlier names, such as itsutsu-ishi, kakugo, gomoku narabe, and goren (五連), into one. As a Renju player, Kuroiwa Ruiko used the pen name Takayama Goraku, and he is regarded as the first Meijin (master).^[1][7][8]

Later, double-three and overline became forbidden for Black only, then a dedicated 15×15 board for Renju came into use, and after that double-four was also added to the forbidden moves. Gomoku was first played on a 19×19 board, the same as a go board, but the larger the board, the greater the advantage for Black, who moves first, so 15×15 became the standard size.^[1][5][8]

In 1966, several separate organizations merged to form Nihon Renjusha as an incorporated association. In 1988, the Renju International Federation (RIF) was founded by 4 countries: Japan, Sweden, the Soviet Union, and the Netherlands.^[2][7]

L. V. Allis showed by computer search (in his 1994 doctoral thesis) that in Gomoku on a 15×15 board with no forbidden moves, Black, who moves first, can always win no matter how White responds, as long as Black keeps playing correctly. Under the rule where only exactly 5 in a row wins, it is also a first-player win. For Renju as well, J. Wágner and I. Virág showed in 2001 that it is a first-player win when no opening rules are used.^[5][6]

Sources

1. Nihon Renjusha (Japan’s renju association), “歴史” [History] (in Japanese) (opens in a new tab)
2. Nihon Renjusha (Japan’s renju association), “概要” [About] (in Japanese) (opens in a new tab)
3. Nihon Renjusha (Japan’s renju association), “ルール” [Rules] (in Japanese) (opens in a new tab)
4. Renju International Federation (RIF), “International Rules of Renju” (opens in a new tab)
5. L. V. Allis, “Searching for Solutions in Games and Artificial Intelligence” (PhD thesis, University of Limburg [now Maastricht University], 1994) (opens in a new tab)
6. International Computer Games Association (ICGA), “Go-Moku Family” (opens in a new tab)
7. Kotobank, “連珠” [Renju] (entries from Nihon Daihyakka Zensho (Nipponica), Sekai Daihyakka Jiten (revised edition), and others; in Japanese) (opens in a new tab)
8. Wikipedia (Japanese), “連珠” [Renju] (opens in a new tab)

Frequently asked questions

What is the difference between Gomoku and Renju?

Renju is Gomoku with forbidden moves (double-three, double-four, and overline) for Black, the first player, only. In standard Renju, Black loses by playing a forbidden move, Black’s first move is played at the center of the board, and in tournaments, opening rules decide how the first few moves are played. On this page, you can choose Freestyle rules with no forbidden moves or Renju rules with forbidden moves for Black (opening rules aren’t included).

What happens if I play on a forbidden point?

Under the Renju rules on this page, on Black’s turn a × appears on each intersection where a move would be forbidden, and clicking or tapping it doesn’t place a stone. A move that makes exactly 5 in a row can be played and wins for Black, even if it also makes a double-three, a double-four, or an overline. White has no forbidden moves, so no × appears on White’s turn. In standard Renju, Black loses by playing a forbidden move.

Does the first player always win at Gomoku?

Computer search has shown that in Gomoku on a 15×15 board with no forbidden moves, Black, who moves first, can always win no matter how White responds, as long as Black keeps playing correctly (L. V. Allis’s 1994 doctoral thesis). It was shown in 2001 that Black can also win at Renju when no opening rules are used. The computer on this page doesn’t know this winning sequence. On Medium and Hard, it looks ahead before each move. On Easy, it doesn’t look ahead and picks from the intersections with the highest scores based on how the stones are lined up.

Does getting 6 or more in a row win?

Under Freestyle rules, both Black and White win with 6 or more in a row. Under Renju rules, White wins with 6 or more too, but 6 or more for Black (an overline) is a forbidden move. Under Renju rules, Black wins by getting exactly 5 in a row.

Why is the board 15×15?

Gomoku was first played on a 19×19 board, the same as a go board. The larger the board, the greater the advantage for Black, who moves first, so 15×15 became the standard size, and the dedicated Renju board is also 15×15. The board on this page is 15×15 as well, with 225 intersections where you can place stones.

How do the computer’s levels differ?

There are 3 levels: Easy, Medium, and Hard. On Easy, the computer sometimes doesn’t block its opponent’s four and sometimes picks moves that aren’t the best. On Medium and Hard, it also searches for sequences of fours that win, and on Hard it uses up to 1.5 seconds per move to search deeper and also defends against its opponent’s winning sequences of fours. When the computer moves first, it plays its first move at the center of the board. The computer’s moves are calculated in the browser on your device.

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