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Bent Four In The Corner
Path: LifeAndDeath   · Prev: CarpentersSquare   · Next: TwistedFourInTheCorner
  Difficulty: Advanced   Keywords: Life & Death, Rules

Bent four in the corner is an expression referring to two things.

  1. The eye shape of a group in the corner being bent four, which we will discuss here.
  2. The fact that under Japanese rules, a group whose eye shape can be reduced to this shape, is ruled dead unconditionally. This rule we will discuss at Bent four in the corner is dead.

1. Less than two outside liberties

[Diagram]
Diag.: Less than two outside liberties

If the defending shape has fewer than two liberties on the outside, Black can prevent unconditional life for White's group and make a ko by playing at 'a'.


[Diagram]
Diag.: Main line

White responds with an atari at 2. Black starts the ko at 3.

White can't play 'a' because of damezumari, and the fate of the group is decided by a ko fight.



2. At least two outside liberties

[Diagram]
Diag.: Bent 4 in corner

If White has an extra liberty on the outside, however, she can play the atari at 4. Black can't fill at 2 because suicide is forbidden.


[Diagram]
Diag.: Bent 4 in corner

So, if Black 5 takes away a liberty, White plays 6 (capturing two stones) and creates life with two eyes.



3. Making bent four in the corner

The position becomes more complicated when the eye shape is almost filled in the following ways.

[Diagram]
Diag.: Situation 1 with a potential bent four

White cannot play at neither 'a' nor 'b' because having her eyespace almost filled with straight three means death. Therefore Black can fill the marked outside liberties at any time, before playing 'a'. As we now know this starts a ko. We now refer to the page Bent four in the corner is dead to explain why this is dead as a rule.


[Diagram]
Diag.: Situation 2 with a potential bent four

Black cannot play at neither 'a' nor 'b' because White will play 'c' to almost fill the eyespace with bent three, which kills. Therefore White can fill the marked outside liberties at any time, before playing 'c' then 'a' or 'b'. Again, as we now know this starts a ko and we refer to the page Bent four in the corner is dead to explain why this is dead as a rule.



Some exercises involving bent four in the corner.


Authors


This page was WikiMasterEdited by Dieter. The previous page can be found at OldBentFourInTheCorner.



Path: LifeAndDeath   · Prev: CarpentersSquare   · Next: TwistedFourInTheCorner
This is a copy of the living page "Bent Four In The Corner" at Sensei's Library.
(C) the Authors, published under the OpenContent License V1.0.