Two players play the game with stones and a board. One player, called Black, uses black stones; the other player, called White, uses white stones. The board is a grid of lines and their intersections.

As a default, the board is a grid of 19 horizontal and 19 vertical lines forming 361 intersections.

Two intersections can be adjacent along a line.

Stones of the same colour are connected if they are adjacent or if there is a chain of adjacent stones of their colour between them.

A string consists of an intersection and any intersections connected to it.

The position is the distribution of black, white, and no stones on all the specific intersections of the grid. For a play, this is given after all its removals.

Move

A move is either a play of one’s own stone on an empty intersection, or a pass.

A play removes stones of every opposing string without adjacent empty intersection.

{no suicide} A play that does not remove stones of any opposing string may not create an own string without adjacent empty intersection.

A move-sequence is a sequence of moves that starts from a position and lets the players alternate moves.

Ko Restrictions

{basic-ko} A play may not recreate the position just before the last opposing move.

{fixed-ko} A play is prohibited if the positions just before and just after it are the same and in the same order as the positions just before and just after an earlier play of the same move-sequence.

Process

The game consists of the following in the given order:

• 1. the alternation
• 2. the black-analysis
• 3. the white-analysis
• 4. the result

The final-position is the position at the end of the alternation.

The black-analysis / white-analysis is a move-sequence that starts by White / Black from the final-position and ends with three successive passes.

Control

In a position, those stones of a player are immortal that are not removed in a move-sequence that starts by the opponent and in that the player always passes.

In the final-position, Black / White controls an intersection if the black-analysis / white-analysis ends with immortal stones of Black / White on the intersection or on all its adjacent intersections.

Result

The score is

• the number of empty intersections that Black controls
• plus twice the number of intersections with white stones on that Black controls
• minus the number of empty intersections that White controls
• minus twice the number of intersections with black stones on that White controls
• plus the number of white stones that are removed during plays of the alternation
• minus the number of black stones that are removed during plays of the alternation.

• a Black win if the score is positive,
• a White win if the score is negative, or
• a tie if the score is zero.