Bent four in the corner

  Difficulty: Intermediate   Keywords: Life & Death, Rules

Chinese: 盘角曲四 (pn jiǎo qǔ s)
Japanese: 隅の曲り四目 (sumi no magari yon moku)
Korean: 귀곡사

[Diagram]
Basic shape  

Bent four in the corner refers to a position that, if played out, will lead to this bent four shape where the "bend" (triangle) is on the 1-1 point. While a normal bent four is a living shape, this shape is unsettled and circle is the vital point. A white play at circle will lead to a living group with 3 points of territory. A Black play at circle leads to a ko favourable to black as described below.

[Diagram]
Position reducable to a Bent four in the corner  

Advanced Topic

A complication is that bent four in the corner is also used to refer to a group where the defender cannot prevent the attacker from reducing it to a bent four on the corner shape, and the attacker can postpone the reducing till the end of the game. Such groups are generally dead, but may live because of unremovable ko threats if played out.

A further complication is that some rules decide the status of this group without having the players to play it out. See bent four in the corner is dead.


Table of contents


Black to play

1. None or one outside liberties

[Diagram]
None or one 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]
Main line  

White responds with an atari at W2. Black starts the ko at B3.

White can't play oshitsubushi at a because it would be self-atari; and the fate of the group is decided by a ko fight.


1.1 Finishing the ko one outside liberty

[Diagram]
Finishing the ko - a  

With a single outside liberty, Black finishes the ko by almost filling the eyespace with a bent three. Playing B1 at a would be inferior as it reignites the ko and gives White a second chance.


1.2 Finishing the ko - no outside liberty

[Diagram]
Finishing the ko - b  

With no outside liberties, Black can finish the ko by capturing all the stones. Playing B1 at a would not be as good, as it leaves White a strong ko-threat.


2. Two or more outside liberties

[Diagram]
Bent four in the corner  

If White has two or more liberties on the outside, however, she can play the atari at W4, an oshitsubushi: Black cannot form a killing shape at W2 because this would be suicide, which either is forbidden or includes self-capture.

[Diagram]
Finishing off  

So, if B5 takes away a liberty, White plays W6 (capturing two stones) and makes life with two eyes.


Reducing to a bent four in the corner

Advanced Topic

Main article: bent four in the corner is dead

A bent four in the corner is also used to refer to a group where the defender cannot prevent the attacker from reducing it to a bent four on the corner shape, and the attacker can postpone the reducing till the end of the game. Such groups are generally dead, but may live because of unremovable ko threats if played out.

Examples

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

[Diagram]
Bent four in the corner  

This position is also known as bent-four in the corner, satisfying the second definition listed at the top of this page. White can 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]
Bent four in the corner  

Just like the previous position, this shape is also a bent four in the corner even though it has a five point eyespace. Similarly to the previous position, Black can fill up White's outside liberties and then proceed to play at a, followed by b. Like the previous position, this bent four in the corner is also dead as a rule.


Avoiding the bent four shape

[Diagram]
Mistake  

B3 at a would be the bent four in the corner. B3 avoids this but at a cost. (Later Black a, White b, and Black has three points.) Although he lives, B1 is a 2-point mistake.

[Diagram]
Correct  

B1 here instead lives with four points, avoiding the bent four altogether.


Corner non-bent four

The following are bent four and are in the corner but are not examples of what is meant by bent four in the corner. In each, a and b are miai for life:

[Diagram]
Not corner bent four 1  
[Diagram]
Not corner bent four 2  
[Diagram]
Not corner bent four 3  

Links

Some exercises involving bent four in the corner

See also


Bent four in the corner last edited by OneWeirdDude on January 5, 2012 - 00:13
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