I have just watched an IGS game, where a ko in a corner position occurred which was a bit confusing for me:

Click Here To Show Diagram Code

[go]$$B
$$ ----------
$$ |.OOX.....
$$ |OXOX.....
$$ |XXOXX....
$$ |..OOX....
$$ |OOOXX....
$$ |XXOOX....
$$ |.XXOX....
$$ |..XX.....[/go]

My first conclusions were

• White can never fill the ko, since then black on b16 would simply kill
• If white plays on b16, then black takes a19 which is a ko for life,
  where black takes first
• If white does nothing, b can eventually take a19.
  • If he wins the ko and fills a18, it's simply a seki
    
  • However if he wins the ko and subsequently plays b16, white takes back on a18 and it's a ko for life, where white took first

In the chat we got into a dispute how this corner would be treated at the end
of the game. If neither white or black does anything, is it considered as seki?

What about, if there is another big seki (= "unremovable ko threat") on the board at the end of the game as in the following hypothetical example (the rest of the board should be considered as settled):

Click Here To Show Diagram Code

[go]$$B
$$ --------------------|
$$ |.OOX..........XOOOO|
$$ |OXOX..........XOX..|
$$ |XXOXX.........XOXXX|
$$ |..OOX.......X.XOXXX|
$$ |OOOXX........XOOXXX|
$$ |XXOOX........XOOOXX|
$$ |.XXOX.........XXXOO|
$$ |..XX...........OOO.|
$$ |..XX...........O...|[/go]

My preliminary conclusion is

• If there is no other seki on the board, the upper left corner should be considered as seki at the end
  • If b plays a19 it ends up with a ko, where w takes first, and since there are no more ko threats left, white should win the ko
    
  • For that reason, b would not start the ko but leave the corner as is
• If there is a second seki, which is smaller than the upper left corner, b can play
  a19 and then play into the second seki as ko thread. White would be alive in the
  upper left corner, but he has to sacrifice the second seki for that purpose
• If there is a second seki, which would be a bigger sacrifice for both sides,
  white can even start the ko on b16, since he has a bigger threat elsewhere, when b
  captures first on a19

So - what do the official rules say about the corner state (apologies, if that position has been discussed before...)?

BTW: In the game, black eventually created a seki by taking a19 and filling a18, which was sufficient to win; thus our dispute remained unresolved...

It's called a 10,000 year ko, some info on it here http://senseis.xmp.net/?TenThousandYearKo

Most rule sets it will be a seki if left alone. There is a little bit about

The ko is called a ten thousand year ko. See http://senseis.xmp.net/?TenThousandYearKo . As the page indicates, a 10,000 year ko was the subject of a rules dispute in the early 20th century. Generally the player with fewer stones at stake has an advantage, and should convert the ko before the end of the game. But the play is not obvious.

In your example, the seki in the top right corner is too big to make a good ko threat. But here is an example with a smaller seki.

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Everything with love. Stay safe.

Thank you for the good explanation! I have read about a 10000-year ko before but I did not really understand it. Watching it in "real life" helped a lot...