So this is the game https://online-go.com/game/13372651 and I was leading by half a point up until C1 was played at move 102. All of a sudden I was behind 0.5 and can’t figure out why this would have an impact on the points. It might of been a different set of rules but I’m not sure how to check the rules after a game has started. What did I do wrong?
You can check the rule-set by clicking on ‘Game Information’ in the right-hand-side pop-out menu.
Move 102 fills up a dame, which does not change the score under Japanese rules (which was used in the game). However, in Chinese rules, a dame is worth 1 point, as the number of living stones of your colour on the board are points for you. The score estimator uses Chinese rules, since it’s easier to estimate score using Chinese rules for a computer, so that might be where you got confused.
Note that at the end of the game black has 37 territory, 11 prisoners on the board and 1 prisoner removed from the board, so 49 points. White has 42 territory, 2 prisoners on the board and 5.5 komi, so 49.5 points.
The reason black lost by 0.5 points is in fact not move 102, but move 103: it takes away one point of territory for black under Japanese rules, while under Chinese rules playing inside your own territory does not affect score directly, so the score estimator did not change. 103 was not necessary to kill the white stones.
As S_Alexander pointed out in chat, actually black could have invaded the top territory as well on move 103 (or earlier), but without 103 that was not even necessary, as white has to protect that point (e.g. with E12, as later in the game), which reduces the white territory by a single point.
The answer lies in the fact that the score estimator, which you were using, estimates things by using Chinese scoring regardless of the rule set of the game. Only the final score is determined by the computer using the game’s rule set, in this case Japanese. White’s move on C1 filled the last dame. This gave W an additional point for the stone, under Chinese rules. However, the apparent loss of a point is only an illusion. If you hand count the score using Japanese rules, you will see that White is already ahead 50.5 to 50 after move 101. Move 102 did not change this. Subsequently, W and B both play one more stone inside their territories, making the final score 49.5 to 49 in W’s favor.
Ok, thanks guys for clearing that up! I didn’t know SE was counting for a differnt rule set.