Bent four in the corner


I’ve reached a bent four in the corner situation in one of my games ( My opponent didn’t realize that, and I want to clear a couple of things from my mind before making my next move.

Under Japanese rules, a bent four is considered dead without playing the ko. If the opponent wants to play it out, one could wait until there are no more ko threats to start the ko. But wouldn’t that cost points? Protecting some ko threats might mean playing inside your own territory.

What is the best way to proceed in such a situation?

Under Japanese rules, bent four in the corner is dead and is removed without playing any ko. This is because the determination of life-and-death at the end of the game assumes that the player of the surrounded group has no unremovable ko threats.; Since the group is considered dead as a rule, you can proceed as if the group were dead (because it is).

PS: The following reply was being typed as I was writing this one.

Hi, According to Sensei’s Library the group is removed without playing any Ko. This is an example of a shortcoming in the Japanese rules, namely, that it contains special provisions rather than a generic rule (The Tromp-Tylor, New Zealand and AGA rules don’t have this shortcoming). The Japanese rules are ambiguous and interpreting them in a straightforward way leads to nosense. When you play ostensibly under the Japanese rules you aren’t actually following the Japanese rule text, for it leaves so many words and situations undefined that an appeal to common sense/practice is required (Which an element outside the ruleset, check the links).

PS: I got the other reply while I was writing mine instants before I posted it.

My question is more about the removal of the ko threats under Japanese rules. By removing all the ko threats (or a suficient number so that the ko is winnable) I’m reducing my own territory. For a very tight game, this could be problematic unless the opponnent accepts the rule and declares the group dead.

Is not every player anyway responsible for removing possible threats in their own territory?

Like it is in my responsibility to secure a stone that is cuttable if my opponent plays on a dame intersection close to it (on the outside)? No matter whether it costs me one point of my territory?

I don’t think so. To protect a cut that is a vulnerability when all dame is filled will be mandatory, but I don’t need to protect a tiger’s mouth connection inside my own territory unless a ko arises. If a ko arises, then things balance out because the opponnent needs to play inside my terriotory before I do, but with the bent four situation, japanese rules assume the group is dead because one can start the ko only after protecting all the threats. But cleaning all that up costs points without the opponent having to play the threat.

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Ah, OK, I understand.

And now you clicked “accept” with that corner alive :open_mouth: while you won anyway, I hope your opponent has read your comments & link to Sensei’s Library, lest they will have a bad surprise in some other game …

I’ve been interested in this every since I had one of these come up on an online game and not get played properly by the player or server.

There is, however, an elegant rule that the dead-four rule stems from. Basically, in Japanese scoring, you finish the game by agreement, then you start scoring through agreement about what is dead. Any dispute is solved by playing out that position to see if the stones die or live WITH NO REGARD TO ANY IMPACT ON THE REST OF THE GAME. You only determine the outcome of the stones in dispute. The board is then returned to the position when the disagreement started, with the stones either removed or not depending on how the resolution phase worked out.

In order for Japanese rules to be properly handled online, the dispute phase would have to be reinstated. Rules like “dead-four” merely stem from this mechanic, which is elegant in its own way. A server properly serving Japanese rules would best go to the mechanic: if there’s a problem in agreement of dead stones, the server asks both parties to click on the stones in question. Then a new board opens, the players play until the stones are removed or until they live. Both players pass here, and are returned to scoring, where they can now agree on the group’s status.