Ikeda's Rule

  Difficulty: Intermediate   Keywords: Rules


This page is about Ikeda's Rule of Scoring in his proposed Area Rules III

  • For 'Ikeda's Proposed Rules of Go', please visit [ext] here
  • For a english translation of Ikeda Toshio's 'On the Rules of Go', please visit [ext] here. When referring to "the rules", Ikeda means "1949? (japanese) rules".
  • For a detailed commentary on Ikeda Territory Rules I by Robert Jasiek, please visit [ext] here

Examples on Ikeda Territory Rules I by Robert Jasiek:

See also /Example 1 and /Example 1 discussion

Ikeda's Rule is a term sometimes encountered when discussing Territory vs. Area Scoring.

To be precise it should be called "Ikeda's Rules" as the term can be traced back to Ikeda Toshio's On the Rules of Go wherein he proposes a number of whole rulesets for Go and not a singular rule. (There are 6 rulesets in the text but only 5 are of Ikeda)

If one speaks of Ikeda's Rule or Ikeda Rules one can never be sure which Ruleset or Rule is spoken of. In these cases it is strongly recommended to mention the corresponding ruleset more precisely.

Nevertheless on this page "Ikeda's Rule" specifically denotes Rule 7 of Ikeda's Area Scoring III in his [ext] Proposed Rules of Go:

  • Rule of scoring: A player's score is the number of that player's played stones plus the number of grid points in that player's territory. If the first pass was made by White, however, then 1/2 point is subtracted from Black's score and added to White's score. The winner is determined by comparing the players' scores.

The following points may help with application of the rule:

  • "If the first pass was made by White, however, then 1/2 point is subtracted from Black's score and added to White's score" Is in practice equivalent to "If the first pass was made by White then add 1 point to White's score" except that with moving a half point from Black to White the total sum of the Black and White scores stays the same. (And that has its beauty)
  • Ikeda's points are moku instead of zi (as some people might expect who do not discern between chinese half-counting and area scoring.)
  • The Rule refers to the very first pass made and not the first pass in the two passes that will initiate the agreement phase. The importance of this distinction becomes obvious when one studies the problem of Pass Fights.
  • Normal territory komi should be used as the rule will compensate White for the one moku more-value of Black's first play if White passed first. (This almost always means that Black got the last competetive(valuable) play and thus one more move than White.)
  • This rule solves the 'free mending in case of even dame' phenomenon and thus presents opportunity for more interesting play.

The first pass

The first pass can also be described as the first pass after the last competitive move, where point where the player decides that playing a stone is not better than letting the opponent play instead.

Because the last competitive move is a subjective concept (the opponent of the player decides which move it is) it can not be used in rules. The pass however can be used in rules, because a pass is an objective fact.

Near the end of the game the player is free to choose between reinforce a group or passing.

  • If he choose to reinforce a group this gives the opponent the option to make the first pass and gain by it. (Black gains by making the first pass because it prevents White from making it)
  • If he chooses to pass, he gains the advantage of having made the first pass but this gives the opponent the option to play on in the normal way (the game has NOT ended) and possibly capture part of the weak group.

End of the game

An other problem of this rule is that it needs a complicated rule for the end of the game:

  • Rule 6 End of the game: After the first pass, the game ends when both players pass in succession.

This looks to mean that if after the first pass nobody places a stone 2 passes need to folow ( a total of 3 passes) But if after a pass stones are placed on the board only 2 passes ends the game. Maybe ending every game with these rules with 3 passes is an option.

Possible uses

One advantage of this rule is that it can also combine territory like scoring with an all stones are alive system a system used in computer go games to prevent discussions about which stones are alive. simply play the game out till all stones are alive.

See also:

--[ext] Nobrowser : I remain in the dark about why Ikeda Rule 7 implies or necessitates his Rule 6. Certainly the English online version of his work sheds no light on this, so can anyone explain?


Ikeda's Rule last edited by on June 14, 2021 - 14:25
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