Ko in Go — The Ko Rule, Ko Fights, and Ko Threats | PlayGo.gg
A repeating situation where both players can keep capturing each other. To stop this from going on forever, one side needs to play elsewhere before taking the 'ko' back.
What is ko?
A ko arises whenever a single-stone capture would, if returned in kind, recreate the board position from one move earlier. The simplest shape: two stones of opposite color sit on adjacent points, each with a single liberty. Whoever captures first leaves a stone the opponent could capture right back — and the cycle would never end.
The ko rule short-circuits this loop. After a capture in a ko shape, the opponent is forbidden from immediately recapturing. They must play somewhere else on the board first. Only after that intervening move does retaking the ko become legal.
When to use ko fights and ko threats?
Once a ko opens, both sides reach for moves big enough to demand a response — these are ko threats. The pattern: capture, threat, response, recapture, new threat, response, recapture, until one side runs out of useful threats and the other connects.
A good ko threat does two things: it forces a reply, and the reply doesn't help your opponent in any other fight. Strong players count threats — yours and theirs — before opening the ko. If you have three large threats and the opponent has two, you'll probably win the ko. If the count is reversed, starting it is a mistake.
Ko shapes matter most at three pressure points: life and death (a single ko can decide a corner), capturing races (the last shared liberty is sometimes a ko liberty), and the territorial endgame (small forcing moves accumulate as ko threats). Note the proverb: there are no ko threats in the opening.
Example of a ko in a game
Where can black play to start the ko?
Ko isn't simply won or lost; it's traded. Count threats before you start, choose your moment, and remember the loser of the ko usually walks away with whatever big move they were threatening — sometimes that's worth more than winning the ko itself.