The history of Gomoku and Renju

A game that came with the Go board

Gomoku exists because Go boards exist. Wherever there is a grid and a bowl of stones, somebody eventually plays a line game on it — and five in a row is the version that stuck, in China, Japan and Korea alike.

Line-making games on grids are ancient and appear in many cultures, but the specific five-in-a-row game on a Go board is documented in Japan from at least the eighteenth century, where it was a popular pastime under names including gomoku narabe — "five points in a row" — and kakugo. In China it is wuziqi (五子棋), in Korea omok.

Renju: the first attempt to fix the game

The competitive problem with Gomoku was obvious to anyone who played it seriously: Black wins. The first player builds threats faster than the second can block them, and between strong players the game becomes a formality.

In 1899 the Japanese journalist and writer Ruikou Kuroiwa proposed a reformed version with a new name, Renju (連珠, "linked pearls"), along with restrictions on the first player. Over the following decades the rules settled into their modern form: Black may not play a double three, a double four, or an overline of six or more.

Renju has been played competitively in Japan since the early twentieth century, and the international body — the Renju International Federation, founded in 1988 — runs world championships with players from Japan, Russia, Estonia, Sweden, China and Taiwan. Estonia and Russia have produced a remarkable number of champions for such a Japanese-sounding game.

Solved — twice, and both times in the first player's favour

In 1993, Victor Allis — the same researcher who solved Connect Four — proved that freestyle Gomoku on a 15×15 board is a win for the first player. His method, threat-space search, is now standard in game AI: instead of searching all moves, search only the sequences of forcing threats, which collapses the tree enormously.

In 2001, Wágner and Virág showed that Renju is also a first-player win, despite the forbidden moves. The restrictions narrow the advantage but do not remove it.

This is why modern tournaments use balancing openings rather than restrictions alone. The Swap2 rule, and the elaborate opening systems used in professional Renju, exist because both earlier attempts to make the game fair were proved insufficient.

What Gomoku gave to computer science

Gomoku's contribution is threat-space search. Because the game consists almost entirely of forcing moves — fours and open threes must be answered — a program can ignore everything else and search only the tree of threats. That idea generalises: any game where large parts of the tree are forced can be searched this way.

The engine on this site works in the same spirit: it classifies every empty point by what a stone there would make (five, open four, closed four, open three, two), then searches threats first. It is the practical version of the same insight that solved the game.

Common questions