Throw-in ko

  Difficulty: Beginner   Keywords: Ko
[Diagram]

Throw-in for ko

In the endgame Black can start a ko here, likely to be profitable since it is a picnic ko from Black's standpoint.[1] First Black plays B1, and White must take with W2.

[Diagram]

Continuation

Then Black plays B3 and White is in no position to connect. White has to start the ko by capturing B3. It is however hard for White to resist: large threats are required.

[Diagram]

Black's ko victory

Black has a follow-up at a which is substantial, once Black has captured to win the ko (marked stone). If White just connects at a Black has of course taken profit in sente here (and then can return to answer White's threat).

[Diagram]

No profit

Simply playing B1 here makes this area all neutral points, which is much inferior for Black.

--Charles Matthews


[Diagram]

Like a 10,000 year ko

Black makes a throw-in at B1 to make the ko. White could make the ko with a throw-in at W2. Black could also make seki by playing B2 instead of B1.

This position is like a ten thousand year ko because whoever makes the ko, the opponent takes it first, and one player can make seki.

Black can change his mind about the seki and make a ko by sacrificing three stones. See Sending Three Returning One.


[1] Bill: Whether this is a picnic ko or not is debatable. It is true that White will have a harder time winning the ko than Black, because it is a sente ko. (Black's winning the ko is a mild sente.) I am not sure if that is enough to qualify this as a picnic ko.
Because it is sente, Black's winning the ko with sente gains nothing (once the ko is made). That is not the same thing as if White has more to lose than Black from the ko.

Charles Looking at what is written at picnic ko and sente ko, I take this to be a sente ko (sense 1); which is called there an extreme case of picnic ko. But Bill comments on the picnic ko page that a true picnic ko results from a tenuki.

Bill: To the best of my knowledge, I was the one to coin the term sente ko some years ago. But since it is just the combination of two common terms, I do not feel that I am the arbiter of its usage. :-) However, I was surprised to hear it called an extreme form of picnic ko.
As for my remarks about tenuki and picnic ko, again, I do not feel that I am an arbiter of usage. I did not mean that all picnic kos arise from tenukis.
The question of picnic ko arises if White wins the throw-in ko, to leave this position:

[Diagram]

Picnic Ko?

(White's last play is circled.)

Now B1 - W2 is sente. In percentage terms, W2 is much hotter than B1, so the ko is rather easier for Black to fight than White. For that reason, can we call this a picnic ko for Black? I think so, but others may disagree.
However, since Black has played tenuki and let White win the original ko, Black may lose his 2 throw-in stones. (He has already lost one; now he is trying to save the second one.) For that reason, we may consider this a picnic ko for White! ;-)


This is a copy of the living page "Throw-in ko" at Sensei's Library.
(OC) 2009 the Authors, published under the OpenContent License V1.0.
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