Ko is the rule that stops a game repeating forever. It is also the source of one of go's most distinctive tactics, where a local fight spreads across the entire board.
Why the rule exists
Certain shapes allow a single stone to be captured, then immediately recaptured, then captured again, with the board returning to an identical position each time. Without a rule, this loops forever.
The ko rule says: you may not play a move that recreates the board position your opponent just left. In practice, after a single-stone capture, the other player must play somewhere else for one move before recapturing.
Recognising a ko
A ko is a single point where each side's capture leaves a shape identical to the one before. The tell is a diagonal arrangement — a single stone surrounded on three sides with the fourth point captured. If you are ever unsure, the board here simply will not allow the illegal move, and the status line says why.
Ko threats: the whole-board part
Because you must play elsewhere for one move, that move becomes a weapon. A ko threat is a move somewhere else that your opponent must answer — a cut, an atari, a reduction. If they answer it, you may return and recapture the ko. If they ignore it and take the ko instead, you carry out the threat.
So a ko fight is really an auction. Each side spends threats, and the side with more threats, or larger ones, wins the ko.
How to play a ko fight
- Before starting a ko, count roughly how many threats each side has. Starting a ko you cannot win hands the opponent a free gain.
- Play your smallest adequate threat first. A threat is only useful if it must be answered; do not waste a large one early.
- Assess whether the ko is worth more than the threat. Sometimes ignoring a threat and taking the ko is correct.
- If you are losing the ko, look for a resolution — a move that settles the position locally and takes compensation elsewhere.
Threats are a resource you can create
Before starting a ko fight, playing a move that leaves a weakness in the opponent's shape banks a threat for later. Strong players build threats before they need them.
When ko decides the game
A ko for the life of a large group is worth everything, and both sides will exhaust every threat on the board. A ko for two points in the endgame is worth very little and should be resolved quickly. Judging the size of the ko before committing is the whole skill.
Other ko situations
Double ko — two kos in the same position give one side an endless supply of threats, which usually makes the group unkillable.
Ten thousand year ko — a ko that neither side wants to start, because starting it is worse than leaving it. Often it simply sits unresolved until the endgame.
Superko — a stricter rule used in some rule sets, forbidding recreation of any previous board position, not just the last one. It closes rare loopholes with longer cycles. The board here uses the simple ko rule, which covers the situations you will actually meet.