group tax

    Keywords: Rules

Group tax is the rules concept, for go scoring, of deducting one point for each eye required for the life of a group at the end of the game. It was used in China before modern area scoring as a convenience in stone counting, and before that, with territory scoring in both China and Japan. You subtract the eyes needed for life from the area, since the player cannot afford to fill those points.


What happens with a seki like this one?

[Diagram]

One eye-no eye-one eye

The surrounding groups have 11 points each, but B has two groups to W's one, so excluding the seki the score is W 9, B 7 with group tax. W wins.

  • Am I correct in thinking that the seki groups are not taxed, as if the game carries on long enough W will kill a B group, liberating the groups in seki?
  • If they are taxed at 2 points/group, then W has two seki groups to B's one, so the game is tied overall.
  • If the tax is one for each eye, then W has two one-eyed seki groups and B has no eyes in the seki, also for a tie.


Another way to look at it: is the tax on seki a matter of the choice of rules, much as group tax itself is?

SiouxDenim

Bill: If the group tax is used with area scoring, as is was in recent history, then each of the White groups in the seki are taxed one point, since they each need an eye to live. The group tax provides a way to do stone counting.

White then gets 25 - 2 = 23 points for the independently alive group plus 5 - 1 = 4 plus 7 - 1 = 6 points for the seki groups, for a total of 33 points.

Black gets 15 - 2 = 13 plus 14 - 2 = 12 points for his independently alive groups, plus 2 points for his seki group, for a total of 27 points.

Ancient go used territory scoring with a group tax. If so, that may be a historical reason[1] why eyes in seki do not count in Japanese scoring.

[Diagram]

Final board for counting?

To give a picture to what Bill is saying: This seems to be the way the group tax takes effect. Now we just count the black and white stones once no more plays can be made, and there's the score. B: 27. W: 33.


[1] Robert Pauli:
Not really, because under Japanese scoring you not even can count false eyes in a seki, but under stone scoring you can:

[Diagram]

Seki with two false eyes

Each side has played 14 stones. Black passes. White adds two further stones and wins by two, just as if White had count his two false eyes under territory scoring.


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