Why is this a ko? (aka how to spot the ko?)

[yebellz]

@le_4TC

Eventually, one side gets to finish the ko, and the other gets some compensation elsewhere.

I want to point out an interesting wrinkle in this statement, which I’m sure @le_4TC is fully aware of and just avoided for the sake of a simpler, clearer explanation, but I’m far more pedantic and feel compelled to say this…

There are some positions where life and death hinges on a ko fight, but one can delay starting it. A peculiar aspect of the Japanese rules is that essentially one player might not need to ever start the ko fight to kill, and, during scoring, after passing, can effectively declare the ko and position settled in their own favor, without having to give compensation elsewhere.

Odd Cases in the Japanese Rules

Special Ko Rules when Determining Life and Death After Passing During the normal course of play, the basic ko rule (which is universal across all go rules) is in effect: Article 6. Ko A shape in which the players can alternately capture and recapture one opposing stone is called a “ko.” A player whose stone has been captured in a ko cannot recapture in that ko on the next move. When a ko arises that both players want to win (e.g., the life/death of a large group may depend on ko), they must …

Under various area scoring rules (like Chinese, AGA, New Zealand, Tromp-Taylor), these positions behave differently, since resolving life and death disputes requires “playing it out”, where compensation could have to be given to settle ko fights (e.g., if there are unremovable ko threats), which might compel a player to never start the fight in the first place.

Life and Death under Chinese Rules

Consider the following board position under Chinese rules with a 5.5 point komi [game1] Which stones are alive or dead? Who will win this game and by how much? What is the best line of play? The purpose of this example is to explore how life and death is handled under Chinese rules. Pause here to consider the above questions if you wish to treat this example as a problem, before reading on into the rest of the discussion. Discussion Note that white’s group in the top-left forms a “bent fo…