Take a Go board and Go stones, throw away the entire rulebook of Go, and keep only one idea: get five stones in a row. That's Gomoku — a game so simple that children learn it in a minute, and so sharp that Japan built a whole professional tradition around taming it.
The rules
- Two players — Black and White — take turns placing one stone on an intersection (or square) of the board. Black moves first.
- Stones never move and are never captured. Once placed, a stone stays.
- The first player to line up five of their stones in an unbroken row — horizontally, vertically, or diagonally — wins immediately.
- If the board fills with no five-in-a-row, the game is a draw (vanishingly rare on a full-size board).
Traditional play uses a 15×15 board (the Renju standard) — which is exactly what RivalPit uses — though the game also works on 19×19 Go boards or any grid big enough to breathe. In free-style rules, rows of six or more ("overlines") also count as wins; some formal rule sets count them only for White.
A game with centuries of aliases
Five-in-a-row games are old enough that their early history blurs. The direct line runs through Japan, where the game — played with Go equipment — became hugely popular by the Edo period under names including gomokunarabe ("five points in a row"), later shortened to gomoku. Related folk games existed in China and Korea as well; wherever Go stones existed, someone eventually lined five up.
In the West, the game arrived in the late 19th century and was sold as Go-bang in Britain and Pegity (a pegboard version) in America — a staple of Edwardian parlours and 20th-century toy cupboards alike. Sweden and Russia developed strong club scenes; Estonia, the Czech Republic, and China dominate much of the modern competitive game.
The problem with going first — and Renju's answer
As players sharpened, an uncomfortable truth emerged: the first player wins with perfect play. Computer proofs (free-style Gomoku was solved in the 1990s) confirmed what strong players had long felt. Japan's answer, developed a century earlier, was Renju: a tournament version where Black — and only Black — is forbidden from making double-threes, double-fours, and overlines. The handicap rebalances the game beautifully and turned five-in-a-row into a professional discipline with titles, rankings, and since 1989 a Renju World Championship.
Casual play — including RivalPit — uses the free rules: no restrictions, first five wins. The first-move advantage is real but only matters between very strong players; for the rest of us, blunders decide games long before theory does.
Gomoku and computers
Gomoku holds a special place in AI history: its huge branching factor but simple win condition made it a classic testbed for game-tree search. Victor Allis's 1993 proof that Black wins free-style Gomoku on 15×15 was a landmark in solving non-trivial games, arriving at the answer through threat-space search — essentially automating the forcing sequences strong human players build by intuition (the same ideas in our strategy guide).
Why it still hooks people
Gomoku sits at a perfect difficulty point: the goal is visible from the first second, every threat is honest and on the board, and yet the game punishes inattention instantly. It's chess's tension without chess's homework — which is why, a thousand years after someone first lined up five stones, we're still doing it.