Real Half Point Ko

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[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.


[Diagram]

Real half-point ko

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

[Diagram]

No ko threats (Black first)

W2 elsewhere, B3 fills the ko at B1.
Result: +1 (for Black).

[Diagram]

No ko threats (White first)

Result: 0.

A (local tally) 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 at B1.

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



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This is a copy of the living page "Real Half Point Ko" at Sensei's Library.
(OC) 2005 the Authors, published under the OpenContent License V1.0.
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