Basic Kofight Example

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  Difficulty: Intermediate   Keywords: Ko
[Diagram]

A ko for life.

Here is an example of a simple ko fight.

Black has just taken a white stone at a. If Black manages to connect at a, his bottom group will live. Although there are only two territory points in it, if his group dies, then White will gain 25 points, so the difference between winning and losing the ko is 27 points. Since that is a difference of 3 plays, each play is worth on average 9 points by miai counting.

White knows this as well so she tries to play a ko-threat.

[Diagram]

White's ko threat

This move threatens to make the white bottom left group alive again (If Black connects the right hand ko, White plays at B2). If this group lives, it will only have two points of territory, but if it dies, Black will get 20 points, so the ko threat is worth 22 points. Responding to the threat has a value of 11 points by miai counting. Therefore, if Black has no good ko-threats, he ought to connect the ko on the right hand side.


Comment: It is not that simple. If Black had no threat he would have to consider the ko exchange (ko no furikawari). If Black fills the ko and White lives, Black gets two points, minus two points for White, net 0. If Black answers and the threat and lets White take and win the ko, Black gets to play one move elsewhere. Locally Black gets 20 points in the bottom left corner minus 25 points for losing the ko, a loss of 5 points. However, if the move elsewhere is worth more than 5 points by miai counting, Black does better (as a rule) than by winning the ko. --BillSpight


However, Black has a series of good threats and is confident of winning the ko, so he responds to the ko threat by playing at B2 himself. White can then retake the ko.

[Diagram]

White takes the ko, Black plays a threat

Of course, Black cannot retake the ko, so Black plays B4 as a threat to stop White from connecting the ko.

If White doesn't respond then Black plays W5 and captures the two marked white stones to the left of his group and achieves his two eyes. A ko threat like this is often referred to as an 'internal' or 'local' ko-threat, because it changes the local situation surrounding the ko. If White responds by playing W5, then Black can retake the ko. If we assume that there are no more ko-threats left, then Black can connect later. He takes two points of territory.

If White doesn't play W5 but connects at a, or plays at b, then Black will play W5, and live with three points ofterritory. However, his two groups are also connected, and although White has gained one point of territory on the right-hand side, Black has gained more than that on the left.

White should therefore answer the ko-threat at W5, and let Black retake the ko:

[Diagram]

White answers, Black takes

In this diagram, White has no more ko-threats.

Black has another local ko threat at B8, so unless White has at least two more ko threats elsewhere, white should just let Black connect the ko on the right.

As can be seen from this sequence, White could not win this ko, since she had fewer ko-threats than Black.

Kiko (5k): So really, assuming White saw she could not win the ko, she should not fight the ko at all. In the given sequence, she loses two/three stones on the lower edge unnecessarily. Like trying to save your stones by playing the first few moves of a ladder, and then realizing that it doesn't work: better not to have started it in the first place!


BillSpight: First, White does not lose any stones on the lower edge. The diagram was confusing. I have corrected it to show that B8 is a ko threat that W9 answers.

Second, there is what is called a ko exchange. White gets something in exchange for Black's winning the ko. Sometimes that occurs when Black ignores White's ko threat. In this case, where Black has more threats, that occurs when White gets an extra play somewhere else.

This ko is worth nine points. Suppose that the ko arises when other plays are worth around seven points. Then we might get this kind of ko fight.

[Diagram]

Ko fight

W4 takes ko. B7 ditto. W8, B9 elsewhere. W10 takes ko. Then

[Diagram]

Ko fight (ii)

W4 elsewhere. B5 fills ko. W6 elsewhere.


Black takes and wins the ko, and gets one move elsewhere, while White gets three moves elsewhere. If these moves are worth around seven points, White gains 14 points in exchange for the ko.

But if White does not fight the ko, but just plays elsewhere after Black takes the ko, leaving the ko until the end of the game, then she will get only seven points in exchange for the ko. At the end she will just fill dame or pass while Black fills the ko.

(Actually, B5 above is a losing ko threat, so White's compensation for losing the ko is somewhat greater.)


Needs WME?


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This is a copy of the living page "Basic Kofight Example" at Sensei's Library.
(OC) 2011 the Authors, published under the OpenContent License V1.0.
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