The Ko Rule in Go, Explained (With Ko Threats)

The ko rule says you may not play a move that recreates a board position that has already occurred. In practice it means that when your opponent captures a single stone in a particular shape, you cannot take it straight back. You have to play somewhere else first, and only then return. Without that rule the same two stones could be captured back and forth forever and the game would never finish.

Ko is the Japanese word for the shape and the fight that grows around it. It comes up in most games once you are past the first few, so it is worth understanding properly rather than memorising. The basic capture rules behind it are on liberties and capture, and the whole rule set is on the rules of Go.

The shape that causes the problem

A ko starts with a single stone that has exactly one liberty, in a shape where capturing it creates the mirror image of what was just there.

A single white stone boxed in by black on three sides with one liberty left at the marked point, surrounded by white stones behind it. This is the standard ko shape.
White's stone has one liberty, at the marked point. Black can capture there.
Black has captured the white stone. The black capturing stone now has one liberty itself, and the marked point is where white would like to recapture but may not.
Black captures. White would love to play the marked point and take the black stone back, but that is exactly what the ko rule forbids.

Look at what would happen if white were allowed to retake. The board would be back to the first diagram, with black to move. Black recaptures, white retakes, and the two players repeat those four moves until one of them gets bored. Nothing else on the board changes. The ko rule closes that loop.

The rule in one sentence

You may not play a move that returns the board to a position it has already been in.

Different rule sets word this differently, and the difference matters in rare cases (see the superko section below), but the practical effect is the same everywhere: no instant recapture in a ko. You must play elsewhere for at least one move.

Ko threats: how a ko is actually fought

Because you have to play elsewhere before retaking, the move you play elsewhere is not wasted. A ko threat is a move somewhere else on the board that your opponent has to answer. While they answer it, your obligation to play away from the ko is satisfied, and you can come back and retake.

Here is the same ko on a 9x9 board, with black having just captured. White cannot retake at the marked point.

A 9x9 board with a ko in the lower left that black has just taken, marked as forbidden for white, and a black two-stone group in the upper right that white can attack.
White cannot retake the ko at the marked point. But black's two stones in the upper right have only two liberties.
White plays a ko threat at 1, putting black's two stones into atari, and black answers at 2 to save them. The ko point in the lower left is still marked.12
White plays 1, putting two black stones in atari. Black has to answer at 2 or lose them.
White retakes the ko at 3 now that a move has been played in between, capturing black's stone. Black in turn may not recapture immediately at the marked point.3
White retakes at 3. Now it is black who cannot play the marked point, and black needs a threat of their own.

That is the whole mechanism. Threat, answer, retake, repeat, until one player runs out of threats big enough to be worth answering or decides the ko is not worth the trouble.

What makes a good ko threat

A ko threat is good when the damage of ignoring it is bigger than the ko itself. Common ones include putting a group in atari, cutting two groups apart, threatening to break into a corner, or attacking the eye space of a group that is not yet settled. Local moves the opponent would answer anyway are the best value, because you lose nothing by playing them.

Two habits are worth building early. Save your threats: a forcing move you play casually in the middle game is a threat you will not have when a ko appears. And check the size of your threat before you use it, because a threat your opponent can safely ignore is just a wasted move that hands them the ko.

Ignoring a threat

Answering is not compulsory. If the ko is worth more than the threat, take the ko and let the threat happen.

White plays the ko threat at 1, black ignores it and fills the ko at 2 to end the fight for good, and white carries out the threat at 3, capturing two black stones.123
Black ignores the threat and fills the ko at 2. White captures two stones with 3. Black decided the ko was worth more than two stones.

That is the real decision in every ko fight, repeated each time: which is bigger, the ko or the threat? Counting is hard at first and gets easier once you know what points are worth, which is what how scoring works in Go is for.

Notice black’s move 2 above. Filling the ko point ends the ko permanently, because there is no longer a single stone to capture. Ending a ko you are winning, rather than fighting it out for extra profit, is usually the safe choice for a beginner.

Positional superko, and what this site uses

The simple ko rule (“you may not immediately recapture”) stops the common case but not every case. There are rare shapes where a longer cycle repeats, with names like triple ko and sending two returning one, and a rule against immediate recapture does not touch them.

This site uses positional superko: no whole-board position may ever be repeated, no matter how many moves ago it occurred, and regardless of whose turn it is. That single rule handles ordinary ko and every one of the exotic cycles at the same time, which means no game can carry on forever and no special case has to be written down. It is also the rule that is easiest to check in software, because the board only has to compare the proposed position against the list of positions already seen.

Japanese rules take the opposite approach: a simple ko rule, plus a provision that a game caught in a triple ko is a “no result” and is replayed. Some Western rule sets use situational superko, which forbids repeating a position with the same player to move. The differences almost never come up in a normal game. Chinese vs Japanese rules compares the whole picture, and the rules of Go covers the rest of the rule set this site plays by.

Advice for your first ko fights

Do not start a ko you cannot afford to lose. Before you take a ko, look at what happens if your opponent ends up winning it, and if that outcome would kill a group of yours, find another move instead.

Count threats before you commit. If you can see two clear threats and your opponent has four, the ko is theirs, so take the local profit now rather than the fight. And when you are ahead and the ko is small, just fill it. Simplifying a position you are winning is a good habit well beyond ko.

Kos are easier to feel than to read about. Play a 9x9 game against the computer and watch for a single stone you could capture that would leave your capturing stone with one liberty. That is a ko forming, and the board will stop you from retaking, which makes the rule obvious the first time it happens. The interactive tutorial also covers it on a real board.

Frequently asked questions

Can you take a ko back straight away?

No. You have to play at least one move somewhere else first, and your opponent has to reply or play their own move, before you may retake. If the board position after your retake would match a position that has already occurred, the move is illegal and this site will not let you play it.

How many ko threats do I need?

One more than your opponent. A ko fight is a threat-counting contest: each side alternates between playing a threat and retaking, so the player who runs out of threats worth answering loses the ko. That is why strong players avoid using forcing moves early, and it is also why a ko can turn a lost corner into a won game if you happen to have more threats on the board.

What is a ko threat worth?

Enough that your opponent loses more by ignoring it than by giving up the ko. There is no fixed number, because it depends on what the ko is worth in that game. A threat that kills a ten-stone group beats a ko worth four points. A threat that gains two points does not.

Try it on the board →