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Three Step Ko
   

Sometimes kos arise in which you have to take the ko more than once before winning it, when your opponent could win it immediately. Each time you have to take the ko is a step of the ko.

Here is a simple Three Step Ko.


[Diagram]
Diag.: 3-step ko

If Black fills the ko the score is 3 points (for Black). If White takes the three steps of the ko with W 1, W 3, and W 5, and then fills the ko, the score is -4 points (i. e., 4 points for White). Each move in the ko gains 1 2/5 points. So the count in the original position is 1 3/5 points.

Very long step kos are possible, but you seldom see one more than 4 steps long.

-- BillSpight

Kiko: I don't understand the math here. 3 points for black vs. 4 points for white means playing the four moves (1, 3, 5, and connect) is worth 7 points. 7/4 = 1 3/4. How do you get 1 2/5?

Bill: If Black fills the ko (1 move), the result is +7. If White takes the ko three times (3 moves) and then fills the ko (1 move), the result is -4. 5 moves make a difference of 7 points. On average, each move is worth 7/5 points.



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