Ko Threat

  Difficulty: Beginner   Keywords: Ko, Go term

Chinese: 劫材 (jié cái)
Japanese 1: コウ立て (kōdate)
Japanese 2: コウ材 (kōzai)
Korean: 패씀 (RR?: pae ssum)

Ko threat may refer either to one of the moves in a ko fight, or to a move which will only be valuable if played during a ko fight.

During a ko fight

After your opponent has taken a ko, any move that threatens a large follow-up may be considered a ko threat. Ideally, the value of the follow-up will be larger than the value of connecting the ko, inducing an opponent to answer the ko threat. If the opponent does answer, it will be legal for you to recapture the ko, and after you recapture it will be the opponent's turn to find a ko threat if he wishes to continue to fight the ko.

If your opponent does not answer your ko threat, but instead ignores the threat and resolves the ko, you can follow through on your threat and thereby gain compensation for losing the ko. Thus while each player would prefer to win without ignoring any (serious) ko threat, a large ko normally ends in an exchange.

Because the value of a ko threat is that the opponent's reply lifts the ko ban and permits the recapture of the ko, it does not matter if a ko threat does not gain points locally. Most ko threats gain little or nothing, other than bringing one player closer to winning the ko. Infrequently, it may even make sense to play a point losing ko threat. Examples of point-losing threats can include playing out broken ladders, wasting aji, and raw peeps.

Example 1

[Diagram]
Example (W4 retakes ko)  

Here is a simplified example of a ko threat. B1 takes the ko. White cannot retake the ko immediately, so she plays a ko threat, the atari at W2. If Black responds, by saving his three stones with B3, White can now retake the ko at white+circle.

Note that W2 at B3 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.

[Diagram]
Example - resolve ko  

In the above diagram, Black could connect the ko with B3, but at the price of giving up three stones after W4. 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

[Diagram]
Example from a pro game  

Here is an example: B1 takes the ko in a ko fight that decides a capturing race in the upper left corner. W2 is a ko threat. B3 decides to resolve the ko. W4 then executes the ko threat, taking the compensation.

Note that Black could also answer W2 by playing B3 at W4. Then W4 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".

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 [ext] 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.

[Diagram]
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.

[Diagram]
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 B1, B3, and B5 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).

More on the /Discussion page.

See also

Basic

Proverbs

Advanced

Discussions


Ko Threat last edited by Dieter on June 25, 2014 - 13:20
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