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J groups
Path: LifeAndDeath   · Prev: TripodGroup   · Next: BentFourInTheCorner
  Difficulty: Beginner   Keywords: Life & Death

Basic J group

[Diagram]
J group

The black shape in the diagram is the basic J group. It has the same status as an L+1 group: it lives or dies according to sente.


[Diagram]
White kills

The correct way for White to kill this shape looks like a complete display of the principal killing techniques: reducing the eye shape with a hane at W1, followed by a placement on the vital point at W3, followed by a descent at W5 and a throw-in at W7. The best way for Black to live is by playing at 3, giving him six points of territory. The order of W1 and W3 is reversible.



The escape with one missing white stone

[Diagram]
Without a, B4 has an alternative

There is one caveat to the above discussion on the J group though. Without a white stone at a, Black can play at B4, aiming to escape at b.


[Diagram]
It's a ko!

White has to respond at W1 (if White responds at W3 instead, Black can start a ko at a), and the result is a ko.


[Diagram]
Comparison

However, with WC, if Black plays B4, W5 will finish the black group off.


[Diagram]
J-group

This version of the J group is definitely dead after W1, W3 and W5. There is no chance for a ko.



The J group with an extra hane

[Diagram]
J group

With the marked hane, Black is alive. There is no ko.


[Diagram]
J group

For example, if White plays W1 and W3, Black lives with B2 and B4. Other details are left for the pleasure of the reader.



[4] Straight J group

[Diagram]
Straight J-group

Black can improve on the status of his corner by descending to the edge instead of making a hanging connection. The best White can do now is turn the corner to ko.


[Diagram]
Ko 1 - White captures first

Here is her best sequence to produce the ko. White gets to make the first capture.


[Diagram]
ko 2 - Black captures first

Here is another way to set up a ko, but this one only works when the black group has no external liberties. Black gets to make the first capture, but if White wins the ko, she removes a part of the black group from the board.



The straight J group with an outside liberty

[Diagram]
Avoiding ko

If Black has an outside liberty, the second ko doesn't work because he can live by answering W1 this way. He sacrifices two stones and plays B10 at B2.


[Diagram]
Avoiding ko (2)

Black is alive in sente for an average gote endgame by white. This result is very favourable for black over being killed straight. If Black had expected 6 points in the corner - he needed to defend the two stones and against the hane - he now has 4 points (B = -2). White on the other hand made 4 points by the two-stone capture (W = +2). As a net loss of six points (B = -2 -4), Black gains hugely in this exchange for not playing gote twice. White might think in terms of "I made 6 points", but she has to realize that she lost around 13 (approx. 19 for capturing the group minus 6 points) points by starting with the wrong move.


[Diagram]
The liberty

When there is no extra outside liberty, as here, W7 instead of a keeps Black short of liberties. Black can't play at b now and must play B8 for the ko.


[Diagram]
Ko 1

The first ko still works with the extra outside liberty.



Practical application

[Diagram]
Not recommended!

Black should under almost no circumstances ignore the double kakari and let White play W1. But how does Black continue if he decides that he has to play this line after all?


[Diagram]
J-group

Black retreats to san-san and White tries to keep his eyespace as small as possible.


[Diagram]
Kikashi before living

Black plays the kikashi of B1 and B3 before living in the corner, in accordance with the proverb.



Discussion

[Diagram]
B4 has an alternative in special cases

Life and death evaluation of shapes is normally done without the ability to escape. Look at the funny white 'wings' on the outside of the L-group in the first diagram of L Group. Even the L-group is alive in sente if descending to either edge is a threat to escape.

The present shape would normally be studied/evaluated with a white stone at b to prevent such an escape. In such a case (no escape) W5 finishes off Black. In studying life and death, then, we need to learn the basic techniques. Next, memorization of the fundamental cases can be a powerful tool to improve play in real games.

However, as much as the vital points and main continuations, we need to thoroughly understand the various conditions such as no escape, no/limited outside liberties, relationship to the corner, etc. that may apply to each case. Small differences in game situations can turn around any of the proverbs/rules of thumb that we have studied so hard!

--DaveSigaty

That's why I try to teach fellow beginners the merit of a net above a ladder. The ladder is a non-local move versus a local move: you need to look at more parts of the goban. Ladders may be easier to pattern-match; sure - but a ladder's status changes more rapidly than a net's.

--JanDeWit

I totally agree with DaveSigaty and JanDeWit on their views. Actually, I have wanted to add the part provided by DaveSigaty after I made the changes but I was unable to get onto this site then.

--unkx80

See also conventions in life and death.



Contributors



Path: LifeAndDeath   · Prev: TripodGroup   · Next: BentFourInTheCorner
This is a copy of the living page "J groups" at Sensei's Library.
(OC) 2003 the Authors, published under the OpenContent License V1.0.