Example ( retakes 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.

Example - resolve ko  

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 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".

Example 2

Example from a pro game  

Here is an example: takes the ko in a ko fight that decides a capturing race in the upper left corner. is a ko threat. decides to resolve the ko. then executes the ko threat, taking the compensation.

Note that Black could also answer by playing at . Then would recapture the ko and the ko fight goes on.

See Ko fight example from a pro game - 1 for the full discussion of this ko fight.

Bad threats and small threats

During a ko fight, a desperate or confused player may make a move which does not actually threaten any serious follow-up. This move may be called a ko threat in virtue of its place in the sequence of the ko fight, especially if the opponent is bluffed into answering it; but it is common to express the judgment that such a move has no working follow-up by saying that it is "not a ko threat".

The same may be said of small threats. A player who has calculated that he can win the game without winning a ko may choose to play a normal move without an unusually large follow-up in lieu of a ko threat. For a player with a large lead, profiting from two such moves in a row may be preferable to the risks of a prolonged ko-fight. Such a move "backs down from the ko" or "abandons the ko".

In addition, some ko threats give the opponent one or more new ko threats. In such cases, playing the ko threat is meaningless or even harmful.

and take ko  

is a harmful ko threat. It gives White two larger ko threats at and a.

Before a ko fight

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." Here, a ko threat refers to a move which is mainly or exclusively valuable when played in a ko. Or, as Charles Matthews says in his article Setpiece Kos, "ko threats are bad moves (plays you'd rather not make)".

When beginners are reminded to kill (or protect) groups in the way that leaves the fewest ko threats, or are told not to waste their ko threats by playing out sequences they know will not work, this is the sense that is intended.

The player with more ko threats available can often play very aggressively, since he is not afraid to start ko fights and can reasonably expect to win any ko that his opponent starts.

A player with vastly more ko threats than his opponent may be able to fight and win a ko without ignoring any of his opponents ko threats, in which case he is a komaster.

Example of a ko threat before a ko fight

This is the L-group. As the proverb says, The L-group is dead.

The L-group  

Black may play whatever moves he wishes in the corner, but White will always have a reply that prevents him from making two eyes.

Wasting ko threats  

For Black to force White to make these moves as soon as Black realizes that his group is dead would be a tremendous waste. The moves , , and are ko threats. So long as Black leaves the corner untouched, he has three ko threats in this corner, and these will come in handy should he need to fight a ko elsewhere.

More terminology

1. It is important to distinguish between local ko threats and non-local ko threats.
2. 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."
3. 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.
4. The value 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).