Renju Rules Explained: Why Black Cannot Play Freely
Renju is competitive gomoku, and the only real difference is fairness: black moves first and therefore starts with an advantage, so renju places extra restrictions on black. This page explains the forbidden moves in plain language.
Why the rules exist
Free-style gomoku is unbalanced. Black moves first, black gets to attack, and with perfect play black wins on a 15 × 15 board — the game was proven to be a first-player win in 1993. Competitive players dealt with this the way players of many games do: give the first player a handicap. In renju, that handicap is a list of shapes black is not allowed to make.
Black's three forbidden moves
1. The overline
An overline is a line of six or more black stones. If black makes six in a row, it is a foul, not a win — even though five of those stones are in a row. Black must win with exactly five.
2. The double four
A “four” is a shape that threatens to become five next move. If one black move creates two separate fours at the same time, that is a double four and is forbidden. Black may create only one four at a time.
3. The double three
A “three” is a shape that can become a four next move. If a single black move creates two separate open threes, it is a double three and is forbidden. This is the rule beginners trip over most, because open threes look harmless.
What white can do
White has no restrictions. White may make overlines, double fours and double threes freely and wins with five or more stones in a row. The asymmetry is deliberate: it compensates black for moving first.
Fouls and losing
In most renju rules a forbidden move loses the game as soon as it is played (or, in some competitive systems, is declared a foul by the referee). Players must therefore check a black shape before playing it — which is why renju is tactically sharper than it looks from the outside.
Opening rules
Tournament renju also restricts the opening to stop players from memorising a winning first-move sequence. Modern systems include the “swap” rules, where the first few moves are placed in a prescribed pattern and players may swap colours to balance the position. You do not need any of this for casual play.
Try it on the board. Open the free gomoku board and switch the Rules menu to Renju: black's forbidden moves are then refused, with a short note telling you which shape — overline, double four or double three — made the move illegal. Leave the menu on Free-style for the casual game most people mean when they say “gomoku”.
Renju vocabulary
| Term | Meaning |
|---|---|
| Renju | The competitive form of gomoku, with forbidden moves for black |
| Overline | Six or more stones in a row; a forbidden move for black |
| Four | A shape that threatens to make five next move |
| Open three | A three with both ends open; can become an open four |
| Double three | Two open threes created by one move; forbidden for black |
Last updated: November 2026