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Carpenters Square
Path: GoProverbs   · Prev: FourIsFiveAndFiveIsEightAndSixIsTwelve   · Next: TheLGroupIsDead
Path: LifeAndDeath   · Prev: JGroups   · Next: BentFourInTheCorner

  Difficulty: Dan level   Keywords: Life & Death

The so called carpenter's square is one of the most complicated of the basic life and death positions. It is simple enough as far as its vital point and status are concerned - it almost always results in a ko. However there is an endless number of ways to set up the ko, and even more ways to defend incorrectly and end up dead anyway.

It is said that anyone who completely understands the carpenter's square must be at least a shodan. My personal interpretation of that gem of wisdom is that I can happily postpone complete study of the shape until I turn shodan, thank you very much.

We'll divide the discussion into an easy part, a difficult part and an extra part.


Easy part

[Diagram]
Diag.: Carpenter's square

The black formation in this diagram is the carpenter's square. The vital point is 'a': if Black plays there, he lives. If White plays there, she turns the corner into ko.



Difficult part

No outside liberties

[Diagram]
Diag.: Basic ko (no outside liberties)

The best way to set up the ko depends to a large extent on the number of outside liberties of the black group. If he has none, White starts by playing on the vital point. Black attaches on the first line, and White extends in the other direction.


[Diagram]
Diag.: Continuation

White further reduces the black eye space and after 5, the only way left for Black to win is engage in a ko.



One outside liberty

[Diagram]
Diag.: One outside liberty

With one outside liberty, White sets up the ko in a different way.


[Diagram]
Diag.: One outside liberty

Black 4 in the previous diagram is forced: here's how Black dies if he plays atari.


[Diagram]
Diag.: The basic method fails

Here's why the basic method fails when there is an outside liberty. Everything goes smoothly for White until 6.


[Diagram]
Diag.: Continuation

However Black squeezes her for two eyes with 2 after she first captures the 'ko'.



Two outside liberties

[Diagram]
Diag.: One outside liberties

If the black group has two outside liberties, White's best way towards the ko is to hane before playing on the vital 2-2 point.


[Diagram]
Diag.: Alternative response

Black can also turn at 2, in which case White needs to continue at the 3-2 point.


[Diagram]
Diag.: Basic attack

The basic attack only produces a two-stage ko that is harder for White to win.



Extra part

Weak carpenter's square

[Diagram]
Diag.: Weak carpenter's square

The weak carpenter's square is a carpenter's square without the connection. The vital point is the same, but this time White kills the corner if she plays on 'a'.


[Diagram]
Diag.: Endgame technique

If Black secured life by playing on the vital point himself, there is an important endgame technique for White to remember: the forcing moves at 1 and 3.



[Diagram]
Diag.: Weak carpenter's square

There's no stone at the marked point, but white a kills nonetheless.

--unkx80



--Stefan



Path: GoProverbs   · Prev: FourIsFiveAndFiveIsEightAndSixIsTwelve   · Next: TheLGroupIsDead
Path: LifeAndDeath   · Prev: JGroups   · Next: BentFourInTheCorner

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