Gomoku, also called five in a row, is an abstract strategy board game. It is traditionally played with Go pieces (black and white stones) on a 15×15 Go board while in the past a 19×19 board was standard. Because pieces are typically not moved or removed from the board, gomoku may also be played as a paper-and-pencil game. The game is known in several countries under different names.
Rules Players alternate turns placing a stone of their color on an empty intersection. Black plays first. The winner is the first player to form an unbroken line of five stones of their color horizontally, vertically, or diagonally. In some rules, this line must be exactly five stones long; six or more stones in a row does not count as a win and is called an overline. If the board is completely filled and no one has made a line of 5 stones, then the game ends in a draw.
Origin Historical records indicate that the origins of gomoku can be traced back to the mid-1700s during the Edo period. It is said that the 10th generation of Kuwanaya Buemon, a merchant who frequented the Nijō family, was highly skilled in this game, which subsequently spread among the people. By the late Edo period, around 1850, books had been published on gomoku. The earliest published book on gomoku that can be verified is the Gomoku Jōseki Collection (五石定磧集) in 1856, although it might have appeared in paintings as early as Heian Jidai, with some folk legend even traces the invention earlier to China. The name "gomoku" is from the Japanese language, in which it is referred to as gomokunarabe (五目並べ). Go means five, moku is a counter word for pieces and narabe means line-up. The game is popular in China, where it is called Wuziqi (五子棋). Wu (五 wǔ) means five, zi (子 zǐ) means piece, and qi (棋 qí) refers to a board game category in Chinese. The game is also popular in Korea, where it is called omok (오목 [五目]) which has the same structure and origin as the Japanese name. In the nineteenth century, the game was introduced to Britain where it was known as Go Bang, said to be derived from the Japanese word goban, which was itself adapted from the Chinese k'i pan (qí pán) "go-board." The game gained some popularity, and in the 1920s it was produced commercially by companies such as The Chad Valley Co Ltd in a modified version marketed as Spoil Five.
First-player advantage Gomoku has a strong advantage for the first player when unrestricted. Championships in gomoku previously used the "Pro" opening rule, which mandated that the first player place the first stone in the center of the board. The second player's stone placement was unrestricted. The first player's second stone had to be placed at least three intersections away from the first player's first stone. This rule was used in the 1989 and 1991 world championships. When the win–loss ratio of these two championships was calculated, the first player (black) won 67 percent of games. This was deemed too unbalanced for tournament play, so tournament gomoku adopted the Swap2 opening protocol in 2009. In Swap2, the first player places three stones, two black and one white, on the board. The second player then selects one of three options: play as black, play as white and place another white stone, or place two more stones, one white and one black, and let the first player choose the color. The win ratio of the first player has been calculated to be around 52 percent using the Swap2 opening protocol, greatly balancing the game and largely evening out the first-player advantage.
Variants
Freestyle gomoku Freestyle gomoku has no restrictions on either player and allows a player to win by creating a line of five or more stones, with each player alternating turns placing one stone at a time.
Swap after 1st move The rule of "swap after 1st move" is a variant of the freestyle gomoku rule, and is mostly played in China. The game can be played on a 19×19 or 15×15 board. As per the rule, once the first player places a black stone on the board, the second player has the right to swap colors. The rest of the game proceeds as freestyle gomoku. This rule is set to balance the advantage of black in a simple way.
Renju Black (the player who makes the first move) has long been known to have an advantage, even before L. Victor Allis proved that black can force a win (see below). Renju attempts to mitigate this imbalance with extra rules that aim to reduce black's first player advantage. It is played on a 15×15 board, with the rules of three and three, four and four, and overlines applied to Black only.
The rule of three and three bans a move that simultaneously forms two open rows of three stones (rows not blocked by an opponent's stone at either end). The rule of four and four bans a move that simultaneously forms two rows of four stones (open or not). Overlines prevent a player from winning if they form a line of 6 or more stones. Renju also makes use of various tournament opening rules, such as Soosõrv-8, the current international standard.
Caro In Caro, (also called gomoku+, popular among Vietnamese), the winner must have an overline or an unbroken row of five stones that is not blocked at either end (overlines are immune to this rule). This makes the game more balanced and provides more power for White to defend.
Omok Omok is similar to Freestyle gomoku; however, it is played on a 19×19 board and includes the rule of three and three.
Ninuki-renju Also called Wu, Ninuki Renju is a variant which adds capturing to the game; A pair of stones of the same color may be captured by the opponent by means of custodial capture (sandwiching a line of two stones lengthwise). The winner is the player either to make a perfect five in a row, or to capture five pairs of the opponent's stones. It uses a 15x15 board and the rules of three and three and overlines. It also allows the game to continue after a player has formed a row of five stones if their opponent can capture a pair across the line.
Pente Pente is related to Ninuki-Renju, and has the same custodial capture method, but is most often played on a 19x19 board and does not use the rules of three and three, four and four, or overlines.
Tournament opening rules Tournament rules are used in professional play to balance the game and mitigate the first player advantage. The tournament rule used for the gomoku world championships since 2009 is the Swap2 opening rule. For all of the following professional rules, an overline of six or more stones in a row does not count as a win.
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