During the opening and middle game under area scoring, a territory count can be used to determine the area count. However, a positional judgement's counting of a basic ko can be difficult to understand at first. The following discusses it during the middle game. Each player has played 17 stones.

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informs about the known territory count and area count values of a basic ko open for Black or White.

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[go]$$B position
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[go]$$B basic ko
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The area counting value of only the two ko intersections is known to be +2/3. The position's area count is 19 - 19 + 2/3 = 2/3.

However, when using the territory count, one must not use the +2/3 value for the basic ko, as explained below.

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[go]$$B territory count
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The territory count of only the basic ko is known to be -1/3 (negative values favour White).

The position's territory count is 3 - 2 - 1/3 = 1 - 1/3 = 2/3.

As one can see, when using the territory count, it determines the area count if the basic ko is evaluated by its territory count!

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[go]$$B explanation
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When considering the area count of only the two intersections of the basic ko itself, then this includes one black stone on the board and its area evaluation. If, however, one considered the territory count for everything except the basic ko but used the area count for the basic ko, then one would determine a value that is 1 point too great. It is the on-board black ko stone's one point, which should provide nothing for the territory count. When considering also this stone's area value, then the difference of black and white stones on all the other intersections is modified. In order to rebalance this difference, one can consider also the marked white stone's area, which is -1. The ensemble of the two basic ko intersections and that one white stone's intersection has the area count +2/3 - 1 = -1/3.

With this calculation trick, we would be correcting the count again: using territory counting for the unmarked intersections and area counting for the marked intersections, we would get 3 - 2 + 2/3 - 1 = 2/3. Obviously, consistently using territory counting for all intersections is easier. (During the late endgame under area scoring, the territory count is insufficient for deciding ko and dame situations. This, however, is a different story than positional judgement during opening and middle game.)

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[go]$$B position
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Area scoring. Each player has played 17 stones.

The area count is 19 - 19 - 2/3 = -2/3.

The territory count is 2 - 3 + 1/3 = -1 + 1/3 = -2/3.

During the middle game, the territory count, applied also to the basic ko's intersections, determines the area count!

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[go]$$B explanation
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Considering the ensemble of only the two basic ko's intersections and the one marked black stone's intersection, the ensemble's area count is -2/3 + 1 = 1/3. Using this and territory counting for the other board intersections, we get the count 2 - 3 + 1/3 = -1 + 1/3 = -2/3, as expected. If only the basic ko were considered with area counting but everything else with territory counting, one would get a value that is 1 point too small. Including the extra marked black stone corrects this, because the stone difference on the other board intersections is rebalanced.

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Conclusion: During the opening and middle game, when using the territory count to determine the area count, use the territory count also for all basic kos!

Is everything correct?