Bent four in the corner
Path: SpecialPropertiesOfTheCorner · Prev: RaceToCaptureInTheCorner · Next: TwistedFourInTheCorner
Bent four in the corner is an expression referring to two things.
- The eye shape of a group in the corner being bent four (with the bend on the 1-1 point), and the associated nakade shape of four stones. When the bent four shape is made, the opponent can play on a 2-1 point to make a ko that she takes first.
- A group which cannot prevent being reduced to a bent four eye shape at the end of the game, nor can it make a bent four earlier to fight the ko before the end of play. Such groups are typically dead, but by area scoring may live because of unremovable ko threats. See Bent Four in the Corner is Dead.
1. Fewer 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.
White responds with an atari at . Black starts the ko at
.
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
With a single outside liberty, Black finishes the ko by almost filling the eyespace with a bent three. Playing at a would be inferior as it reignites the ko and gives White a second chance.
With no outside liberties, Black can finish the ko by capturing all the stones. Playing at a would not be as good, as it leaves White a strong ko-threat.
2. At least two outside liberties
If White has an extra liberty on the outside, however, she can play the atari at , an oshitsubushi: Black cannot form a killing shape at
because this would be suicide, which either is forbidden or includes self-capture.
3. Making a bent four in the corner
The position becomes more complicated when the eye shape is almost filled in the following ways.
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.
3.1 Bent five in the corner ?
This shape is also known as a bent four in the corner although it has a five point eyespace. The reason is that bent four refers to the nakade shape that White makes with four stones when she almost fills Black's eye, and, thus, to the shape of the eye if Black captures those stones.
![[Diagram]](../../diagrams/11/78dec6ea82fae1633f895f16d9f7f719.png)
Black is in atari
White can fill Black's outside liberties and then make a bent four shape while putting Black in atari If Black captures we get a four space bent four in the corner. with no outside liberties. which is explained above.
4. Avoiding The Bent Four
at a would be the bent four in the corner.
avoids this but at a cost. (Later Black a, White b, and Black has three points.) Although he lives,
is a mistake.
5. 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:
6. Links
Some exercises involving bent four in the corner
See also
Path: SpecialPropertiesOfTheCorner · Prev: RaceToCaptureInTheCorner · Next: TwistedFourInTheCorner