4. Gameplay
Making a Move
- Players alternate. A move is either placing one stone of your colour on any empty intersection, or passing (AGA Rule 2)
- Passing is always legal, even when legal placements exist
- Stones do not move once placed — they are only removed by capture
Liberties, Strings & Capture
- Adjacent stones of the same colour form a string and share their liberties
- A liberty is an empty point orthogonally adjacent to a stone in the string
- A string with no remaining liberties is captured and removed from the board as prisoners (AGA Rule 5)
- Suicide is illegal — you may not play a stone that would leave your own string without liberties, unless the play captures first
The Ko Rule
- It is illegal to play so as to recreate a previous board position from the game with the same player to move (AGA Rule 6)
- This is a positional superko: it covers the classic single-point ko and also longer repeating cycles
- In practice a player facing a ko must play elsewhere (a ko threat) before recapturing
Passing
- A player who passes gives the opponent a pass stone, which counts as a prisoner (AGA Rule 7)
- Pass stones are what keep AGA territory scoring and area scoring in agreement
- Under AGA rules White must make the last pass ending the game