Every board game with captures needs a rule against endless repetition. In Go that rule is called ko, and it produces some of the most exciting moments in the game. Beginners usually meet ko by accident: they capture a stone, the opponent immediately tries to capture back, and the computer refuses the move. This article explains why, and how to turn ko from a confusing rule into a weapon.
The ko shape
Ko appears when a single stone captures a single stone, and the capturing stone is itself left with only one liberty — the point where the captured stone was.
Look at the position after Black plays f5 and removes the white stone at e5. The new black stone at f5 is surrounded on three sides by white stones (g5, f6, f4) and its only liberty is e5, the point just emptied. White could play e5 and capture it at once — which would bring the board back to exactly the shape before Black's capture. Then Black could capture again, and so on forever.
The rule itself
The ko rule forbids this loop: you may not make a move that recreates a previous board position.
The version used on this site is positional superko: the whole-board position after your move must never have appeared before in the game. In ordinary ko this simply means White cannot recapture at e5 immediately. White has to play somewhere else first. If Black answers that move instead of resolving the ko, the board is now different, so White is allowed to take back.
Several traditional rule sets state ko slightly differently, but for everyday play the effect is identical. The superko version only matters in rare positions with several kos on the board at once.
Ko threats
Because you must play elsewhere before retaking, the natural question is: where? The answer is a ko threat — a move that demands an answer. For example, a move that puts a big enemy group in atari, or threatens to cut off and capture something valuable.
A ko fight goes like this:
- Black takes the ko.
- White cannot retake, so White plays a threat somewhere else.
- Black must decide: answer the threat, or end the ko by filling or connecting it.
- If Black answers, White retakes the ko, and now it is Black who needs a threat.
The fight continues until one side runs out of threats worth answering, or decides the ko is not worth the threats. Then the other side resolves the ko and collects the prize.
How to judge a ko
Before starting or accepting a ko fight, ask two questions.
What is the ko worth? Some kos decide whether a large group lives or dies. Others are worth a single point in the endgame. Count the difference between winning and losing it.
Who has more threats, and bigger ones? Look around the board for moves that force answers. Each side's threats are limited. Using a threat also usually costs something: if your opponent ignores it, you get to carry out the threat, but they get to win the ko.
A useful simple rule: if the ko decides the life of a big group, and you have fewer threats, avoid starting it. If you have many threats and the ko is large, a ko fight can turn the game around.
Beginner mistakes with ko
- Trying to retake immediately. The site will not allow it. Remember: play a threat first.
- Using a threat that is not really a threat. If your opponent can safely ignore it, you have wasted a move and lost the ko.
- Making threats that damage your own position. Some threats force an answer but also destroy your own potential or give away points.
- Ignoring small kos in the endgame. Near the end, a one-point ko still matters when the game is close. Resolve kos you are winning; do not leave them open.
Kos that are not simple
Beyond the basic shape, Go has more complicated kos: approach kos, where one side needs extra moves before the ko is really a ko, and positions with several kos at once. Beginners rarely need to worry about them. The most common special case is a ko where only one side risks a big loss; strong players call that a "picnic ko" for the other side, because they can keep fighting it at little risk. You will learn to recognise these naturally as your reading improves.
Ko and passing at the end
Since the game on this site ends after two passes and every stone on the board counts, make sure no ko is left open where your opponent could still capture something valuable. If a ko remains, resolve it (connect or fill) before you pass.
Practise ko safely
You do not need a special drill. Play some 9 × 9 games against the computer on the home page and watch for single-stone captures along the edges and in shapes like the one above; kos appear often on small boards. When you play real opponents on the online page, sign in on your account so the game is saved, then replay any ko fight afterwards and count how many threats each side had.
For the full basic rules, see Go rules for beginners; for related illegal moves, see suicide and illegal moves.
A tiny example of counting threats
Suppose a ko decides whether a black group of eight stones lives. If Black wins the ko, the group lives; if White wins, it dies. That is a big ko. Now look for threats: Black has a move that puts a white corner stone in atari, and another that threatens to cut a white group. White has one move that threatens a black group on the other side. Black has two threats, White one — Black should expect to win the ko if both sides play well.
Frequently asked questions
Why can't I take back the ko immediately?
Because the board would return to the exact position before your opponent's capture. The ko rule forbids repeating a position, so you must play elsewhere first.
What exactly is a ko threat?
Any move that your opponent should answer, such as putting a group in atari or threatening a large capture. It is played so that, when your opponent answers, the board has changed and you may retake the ko.
What is superko?
A version of the ko rule that forbids repeating any previous whole-board position, not just the immediately previous one. It is the rule used on this site.
Can a game end while a ko is still open?
Yes, if both players pass. But on this site the final count is literal, so leaving a ko open can cost you. Resolve it before passing if it matters.
Are ko fights common in beginner games?
Simple kos appear often, especially on the edge. Big ko fights become more common as players learn to read and to find threats.
