I knew about bent four since decades. I believe I have really understood it only recently. (But who knows, maybe that moment is still in the future.) It didn’t happen in my games very often yet though.

We have a thread for sharing sekis already, so why not one for bent fours?

I’ll make a start: here is one from a recent game of mine involving around 45 points in the lower right corner.

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Did anyone have 2 bent-four in 1 game?
So only one of 2 groups can be actually killed with it if both are same color.
And I can’t imagine what happens if they are opposite color.

(There is bureaucracy instead of mechanics in Japanese rules, so I’m talking about Chinese)

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I have a thread where I work through how bent-four works under Chinese rules. Basically, the bent-four is dead in ko, which means that if there are big enough, unremovable ko threats, it could actually live.

Another example is in the last post of that thread:

Under Japanese rules, if one player has two bent-four formations in separate areas, then they would (most likely, unless some other weird connected things are happening) both be dead groups. The life and death resolution phase of Japanese rules applies special ko rules that would essentially isolate each position and nullify any ko threats. See:

Two bent-fours in opposite colors could actually result in a seki (under both Japanese and Chinese rules):

Unfortunately, the website that I linked there seems to now be down, but basically, it showed how the position is a hanezeki-like oddity, where it behaves like a strange capturing race, with the person who starts it would lose.

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Found the example and posted about it here:

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I was actually just looking at this yesterday after encountering bent four in one of my games. I was curious how the different rules handled it and encountered the bent four seki under all rulesets.

Here’s a fun position, not quite bent four, but pretty closely related:

What is best play for black/white playing first under chinese rules?
(I have no idea about japanese rules, maybe @yebellz can analyze that for us!)

Although the position is presented on a compact small board here, it does not in fact rely on the two groups being adjacent (at least in chinese rules). This sort of situation could happen in a normal 19x19 game, but I don’t know of any examples from real games.

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Ah its because if white plays first he dies. If black plays first it gives white a ko threat. White is inevitably alive in this position after all ko threats are removed.

(i.e A1 A4 A2 E4 F4 B2 making the KO and A3 miai)

If white play’s first he will just lose the ko and die if there are no ko threats.

Edit: but, I don’t know how this would play out under the rules.

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That’s a nice position. If I’ve understood it correctly, I believe that this is one of the cases that behaves very differently under the Japanese rules.

I’ll have to think about it a bit to make sure, but I’ll eventually post an analysis in my Odd Cases thread.

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Under Chinese rules, both players should pass, and all stones are alive in seki.
Under Japanese rules, both players should pass, but all White stones are dead (and removed), and all Black stones are alive*.

*Even though it seems redundant, I’m pedantically saying that all of the Black stones are alive and that the dead White stones are removed, just to clarify that there are not any other weird situations going on, like dead stones not being removed due to anti-seki or residing in the eye of a seki.

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