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Sending Two Returning One
    Keywords: Rules

An aspect of territory scoring is that one player may have a way to prolong the game while losing points. (See example on the Tsumego From Games 21 page.)

Under area scoring a play inside territory to prevent this would cost nothing, right at the end of the game. That isn't the case under territory scoring; it may be practical to give back one point eventually.


Robert Pauli:

Actually it's area scoring that has a problem with "sending two, returning one".

[Diagram]
Send 2, return 1

Without some super ko rule, this 4x3 game wouldn't end:

Black will endlessly attack instead to lose by 6 - 4 = 2 points.

Under territory scoring, on the other hand, White (leading by, say, 1 prisoner) would simply give up the whole board at the moment she piled up enough black prisoners - 26, I guess. :-)



[Diagram]
After suicide

In case suicide is an option and Black commits it (clearing the whole board), the game will continue as shown, forcing Black to pass with B10. White will pass too - if she doesn't want to make fun and start "sending two, returning one" herself. ;-)


Bill: Example, please. I do not believe I have ever seen a sending-2-returning-1 position that could be prevented by a play inside one's territory, except by preventing the seki in the first place, which would be a sizable play under either scoring style.

Charles I was thinking of something like this.

[Diagram]
White is dead but ...

White at a is a threat (to make seki) so that Black must capture two; White then can recapture one to repeat the board position. If Black eventually plays b that prevents the repetition.

unkx80: Uh, White a is a threat to make seki? Isn't it a direct atari?
Charles Matthews Ah yes - with another external liberty for Black it would threaten seki.
Bill: Thanks, Charles. Interesting position. But the threat to prolong the game is slight.

[Diagram]
Dead as Elvis

W3 at the circled stone.

Now Black can play at B4 with no net loss, regardless of scoring method.



There may be no such play, as in the case just cited and also in the example on the superko page. Then in principle under territory scoring one allows the opponent to do it 100 times (say) and then passes instead of continuing round the cycle, allowing the opponent to take a few stones.

Under a superko rule such positions cannot be repeated indefinitely.


Side note:

The phrase sending two returning one is a direct translation of 打二还一, which describes the sequence below and is used to differentiate it from ko when teaching Go rules to beginners. --unkx80

[Diagram]
Sending two returning one.

[Diagram]
Sending two returning one.

[Diagram]
Sending two returning one.

W2 is a legal move even if played immediately after B1.



This is a copy of the living page "Sending Two Returning One" at Sensei's Library.
(OC) 2003 the Authors, published under the OpenContent License V1.0.