The situation in go in which immediate recapture of a single stone is forbidden and one must therefore make a threat in order to be allowed to do so. This tag is definitely appropriate to questions about the principles and rules of kos and about a ko in a particular position. For positions that might lead to a ko and positions where a ko is a small part it is probably only useful if the ko plays a significant role.
A ko is a situation in go in which immediate recapture of a single stone is forbidden and one must therefore make a threat in order to be allowed to do so. A ko threat is such a threat, and a ko fight is the series of threats answers and recaptures until the situation is settled.
The basic ko position
$$ Ko $$ --------- $$ ........- $$ ..XO....- $$ .Xa1O...- $$ ..XO,...- $$ ........-
In this position Black has just captured a stone at a; White would like to recapture, but this is forbidden. In this case White needs to find another position on the board in which he can threaten to take enough points off Black to make them answer, after which White is allowed to retake and Black is the one needing a threat. In the end, unless one player runs out of threats, whoever wins the ko fight (e.g. by connecting) will have had to accept the loss due to ignoring the last threat.
Varieties of ko
Kos come in several varieties apart from the simple one above:
- Two-step ko, where at least one side must ignore two threats to win.
- Two-stage ko, where two linked kos segue into one another and at least one side must ignore two threats more than the other to win.
- Multi-stage or -step ko analogous to two-stage and -step ko
- Thousand-year ko, where neither player wants to start it, as doing so makes it much harder for them. (Starting is normally an advantage, as it forces the opponent to find the first threat.)
- Double ko, where the move needed to finish one ko may be prevented by taking another ko.
Creating a ko
When considering how to create a ko, various things are important:
- It is desirable to make the first capture, so your opponent has to make the first threat.
- The fewer threats a player must ignore to win, the better for them.
- You should ideally know how many threats you and your opponent have.
Choosing and answering a threat
The question when a ko threat should be answered is complex: it is not simply the case that one has to answer only those which threaten a larger loss than that due to losing the ko and no others. The order in which threats should be made is also tricky. For details, see the article on ko threats and the rule of thumb in Sensei’s Library.
The rules of ko
Although all versions of the rules forbid retaking a ko, the formulations vary with different effects in a few unusual situations.
The theory of ko
The existence of kos means that analysis of go with combinatorial game theory needs the concept of loopy games.
For more detail on kos, see the article on ko and related articles in Sensei’s Library.