Bent four in the corner is dead
This page discusses the fact that under Japanese rules, a group which is surrounded by a live enemy group and whose eyeshape can be reduced to bent four in the corner is dead.
Note that the words can be in the above sentence are very important. If Black plays at the marked point, then White can capture at b and either make a ko (if there are 0 or 1 outside liberties), or live via oshitsubushi.
Let's talk over this position from bent four in the corner.
1. Japanese rules
By Japanese rules the white group in the above diagram is considered dead and is removed without playing any ko. The reason for this is as follows.
Since White can never play a nor b, Black could fill all outside liberties after play has stopped at the end of the game, and then play a to create the ko explained in bent four in the corner and shown in the following diagram:
The current rule is now that in the determination of life and death at the end of the game, after play has stopped, only a pass for a specific ko__ will allow that ko to be taken back. If White makes such a pass, Black wins the ko, anyway.
The first written Japanese rules, in 1949, had a special rule that "bent four in the corner is dead". This ruling has been discussed ever since, because of the fact that some[1] games contain unremovable ko threats. Many believed that that made this rule unfair towards the defending side in this position.
2. Other rulesets
Under Chinese rules or under rulesets where life and death questions may be settled by actual play rather than decided by virtual play, the ko should be played. Such rulesets provide means to remove dead stones by playing inside your territory after the game without losing points (e.g. by using area scoring or pass stones). Thus, it costs nothing to remove all ko threats before starting the ko, so the bent four in the corner is still essentially dead. However, unremovable ko threats can make a difference.
3. Caution even under Japanese Rules
- When the surrounding group of a group which is supposed to be dead by this rule is not alive with two eyes itself, the rule doesn't apply: in that case the aforementioned ko will be played out. [2]
- Japanese rules can be difficult to apply, e.g. in connected Bent-4 situations -- Example at
Undead-Bent-4
4. Bent-4 on the Go Servers
Could someone write an overview how Bent-4 is treated on the different Go Servers?
It is one thing to say that one would "apply Japanese Rules" and another to bring them into practice via an electronic medium, most probably without a referee. Which server does actually have this phase where 'only a PASS is allowed as Ko threat'?
My understanding of Japanese rules on go servers is that they are simply not implemented properly, at least on kgs. I have personally lost a game because it was played under japanese rules and my opponent had a shape that would become bent four once played out. The dead shape was disputed by my opponent and I was faced with the choice of either filling in all of my opponent's ko threats and playing it out, or just playing it out and ignoring a threat. Either option would lose the game by about ten points.Under chinese rules, I would have simply removed the ko threats, without losing any points, and then killed the bent four. The worst part about it is that on KGS Japanese scoring is used by default and few people bother to use area scoring, so this situation must be extremely common
-- Anonymous
zinger: I think that "extremely common" is an exagerration - I have played over 2000 games of Go and never once had a disputed bent four. Besides that, it sounds like the problem is not that KGS is implementing the rules improperly, but that the opponent did not understand the rules. After all, a dishonest opponent can wrongly dispute life and death under any ruleset.
Some rulesets do not have the notion of "wrongly" disputing life and death. There is no such problem in Kurnik which uses AGA rules. -- Tomek Czajka
KGS does implement rules improperly, especially those titled "Japanese Rules" on this server. The programmer has told me: One should occupy all non-seki-dame because KGS makes scoring mistakes otherwise (it is well known that KGS does indeed); where the players cannot agree on scoring, they are supposed to ask a third person to arbitrate; needless to say, implementing and executing complete Japanese rules is theoretically impossible; there are several "Japanese Rules" rulesets, so a server cannot implement them all at the same time under just one title. To describe all this, I speak of "KGS-Japanese Rules". --RobertJasiek.
See also
Authors
- Andre Engels
- HolIgor
- Juha Nieminen
- Karl Knechtel
- Dieter (who also did a WikiMasterEdit)
- tderz
[1] Many games in number, but a tiny fraction of all games.
[2] Well, it can be more complicated than that too, but I think I'll leave that for some other Robert.
Harleqin: I won't say it is complicated. The rule only applies after the end of the game. If the attacker is forced by circumstances to actually remove the "dead" stones before the end of the game, then he will obviously have to fight the ko.
Bass: Interestingly, if the surrounding group has only one four-space big eye, then the same rule works in exactly the opposite direction, and the bent four in the corner is unconditionally alive. See suicidal tendencies for an example.