Ko Threat (version 103)
A ko threat is a move with a "big" follow-up, which you play after your opponent has captured a ko; if your opponent answers it you may recapture the ko. (You cannot recapture the ko immediately, due to the basic rule of ko.) If your opponent does not answer your ko threat, but instead proceeds to resolve the ko, you can follow through on your threat and thus gain compensation for losing the ko.
The basic sequence of a ko fight is thus (1) White takes ko (2) Black makes threat (3) White answers threat (4) Black takes ko (5) White makes threat (6) Black answers threat (7) White takes ko, and so on until finally (3n) Black or White ignores threat to resolve the ko (3n+1) White or Black follows up on threat.
The term ko threat can be used to refer to a potential threat as well as one actually played. Example: "White has more ko threats than Black."
The term "ko material" is an alternative to ko threat, which can also refer to the aggregate of all potential ko threats one player has, as in "Felix has more ko material than Oscar." The Japanese terms kouzai and koudate, do not distinguish between singular and plural, and thus may be used to refer either to ko material in the aggregate or an individual ko threat.
The "size" of a ko threat is, informally, the potential gain from making the threatened follow-up (when your opponent ignores the threat and instead resolves the ko).
Here is a simplified example of a ko threat. takes the ko. White cannot retake the ko immediately, so she plays a ko threat, the atari at . If Black responds, by saving his three stones with , White can now retake the ko at .
Note that at is another possible ko threat. No one can say which is better in isolation. Where there is a choice of ko threats to play, you have to look at other factors.
In the above diagram, Black could connect the ko with , but at the price of giving up three stones after . It is up to the players to decide whether this exchange favours Black or White.
Therefore, the size of the available ko threats is also an important factor in ko fights. In the Go literature, sometimes a ko threat whose value is smaller than the value of the ko itself is described as "not a ko threat".
Ko threat strategy
Before the fight
- Count ko threats.
- In anticipation of an upcoming ko fight, consider creating potential ko threats.
- Or, by the same token, find ways to eliminate ko threats on the part of your opponent
(especially local ko threats (see an example of removing a local threat), and double ko threats?, meaning they work as ko threats for both sides).
During the fight
- If you have one or more ko threats "big enough" (threatening enough damage), so that your opponent should answer them, play the smallest one.
- If you will win the ko at your next opportunity, and a new ko begins or threatens to come about while the ko threat situation remains the same, you will be glad you only played your smallest effective threat.
- Otherwise, play the biggest one you have.
- If you will lose the ko, you want to get the most you can in exchange for it.
- Try to avoid ko threats which lose points--loss-making threats.
- Try to make ko threats which are moves you would have liked to play anyway.
- Your ko threat needs to be a real threat; otherwise, it's called mukou.
- You should give absolute priority to local ko threats, which threaten to resolve the local situation in your favor regardless of the outcome of the ko, and which your opponent therefore has to answer at the risk of making the ko meaningless.
Note: There may be exceptions to the above advice. Whether to play a ko threat, and if so, which one, can be very subtle questions. See ko threat playing order for more.
- In general, when considering moves take into account the ko threat implications. Favor situations which give you more ko threats and your opponent fewer.
More on the /Discussion page.
- ko threats - basic?
- remove double threats before you first capture the ko
- double ko threats?
- capture once to use up a threat
- Ko threat counting exercise? and Practical Endgame Test 4