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Ko
HalfPointKo

 

Real Half Point Ko
    Keywords: Ko

[Diagram]
Diag.: 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.

[Diagram]
Diag.: Real half point ko

However, this ko really does have a miai value of 1/2. :-)
Unless Black is komaster, that is.


[Diagram]
Diag.: No ko threats (Black first)

W 2 elsewhere, B 3 fills.
Result: +1 (for Black).


[Diagram]
Diag.: No ko threats (White first)

Result: 0.

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

[Diagram]
Diag.: White komaster (White first)

W 3 - B 4 = ko threat and reply. W 5 takes ko back. B 6 elsewhere. W 7 at 2.

Result: -1.


[Diagram]
Diag.: White komaster (Black first)

W 2 elsewhere. W 4 - B 5 = ko threat and reply. W 6 takes ko back. B 7 elsewhere. W 8 at 3.

Result: 0.

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

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

[Diagram]
Diag.: Black komaster (White plays)

W 1 fills, leaving a 1/2 point play at a.
Result: -1/2.


[Diagram]
Diag.: Black komaster (Black plays)

W 4 takes ko. B 5 - W 6 = ko threat and reply. B 7 at 1.

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

After B 1 the local temperature drops to 1/3. Black has a sente sequence with B 3 - W 8.

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

-- Bill Spight




This is a copy of the living page "Real Half Point Ko" at Sensei's Library.
(C) the Authors, published under the OpenContent License V1.0.