Real Half Point Ko

  Difficulty: Advanced   Keywords: Ko

Table of contents Table of diagrams
So-called half point ko
Real half point ko
White wins the ko
Black wins the ko
Two points
Real half point ko (2)
No ko threats (Black first)
No ko threats (White first)
White komaster (White first)
White komaster (Black first)
Black komaster (White plays)
Black komaster (Black plays)

So-called half point ko

[Diagram]
So-called half point ko  

This ko, commonly known as a half point ko, has a miai value of only 1/3.

As a rule, there will be no smaller plays between this ko and dame, so that the only question is who wins the ko, and it will behave as though it were a half point ko.


Real half point ko

[Diagram]
Real half point ko  

This ko really has a miai value of 1/2.

[Diagram]
White wins the ko  

If White wins the ko White gets 3 points of territory locally.

[Diagram]
Black wins the ko  

W2 elsewhere

Black wins the ko with B3, unless that is ko-banned in which case Black wins the ko with black+circle. Either play is sente and White responds at W4.

[Diagram]
Two points  

In the end White will net only 2 points of territory locally.

A net difference of two plays in the ko makes a net difference of one point. Each play has a miai value of 1/2.


Real half point ko, komaster dependent

[Diagram]
Real half point ko (2)  

This ko also has a miai value of 1/2, unless Black is komaster.

[Diagram]
No ko threats (Black first)  

W2 elsewhere, B3 fills the ko at white+circle.

Result: +1 (for Black).

[Diagram]
No ko threats (White first)  

Result: 0.

A difference of two moves makes a difference of one point, so the miai value of the ko is 1/2.

[Diagram]
White komaster (White first)  

W3 - B4 (ko threat) and reply. W5 takes the ko back. B6 elsewhere. W7 at W2.

Result: -1.

[Diagram]
White komaster (Black first)  

W2 elsewhere. W4 - B5 = ko threat and reply. W6 takes ko back. B7 elsewhere. W8 at B3.

Result: 0.

Again, a difference of two plays makes a difference of one point. Miai value of ko = 1/2.

When Black is komaster the ko has a miai value of 7/12.

[Diagram]
Black komaster (White plays)  

W1 fills, leaving a 1/2 point play at a.

Result: -1/2.

[Diagram]
Black komaster (Black plays)  

W2 elsewhere. W4 takes ko. B5 - W6 = ko threat and reply. B7 takes ko.

Result: +2/3. (Black has taken one stone for one point, minus 1/3 point for the black stone in ko.)

After B1 the local temperature drops to 1/3. Black has a sente sequence with B3 - W8.

Two plays make a difference of 1/2 + 2/3 = 7/6 points. The miai value of the ko is 7/12.

-- Bill Spight


See also:


Real Half Point Ko last edited by RickyDemer on September 11, 2018 - 15:21
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