Carpenter's Square

The so called carpenter's square is one of the most complicated of the basic life and death positions. It is said that anyone who completely understands the shape is at least professional shodan. It was misunderstood until the 1300s. It is simple enough as far as its vital point and status are concerned - it almost always results in a ko. However there are endless ways to set up the ko, and even more ways to defend incorrectly and end up dead anyway.

"See Also" section has links to closely related shapes.

Table of contents Summary Details No outside liberties One outside liberty See also Book references Online references

Table of diagrams [Carpenter's Square] Basic 2-2 attack Continuation Throw-in ko Alternative: clamp Variation White's option One outside liberty One outside liberty The basic method fails Continuation

Summary

Carpenter's Square  

The Black formation in this diagram is the carpenter's square. White a to c all lead to ko. When Black has one or more outside liberties, only a works.

d is a trick play: it's hard to find the correct defense leading to unconditional life.

Black to play can live with almost any move.　Usually a is chosen to make both of Black hane at c gote in the endgame.

Details

No outside liberties

Basic 2-2 attack  

a) 2-2 placement is the basic way, because it works even when Black has outside liberties or outside hane at a. Black must attach on the first line. Now White has many ways to start a ko, for example by bump in the diagram.

Continuation  

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

Throw-in ko  

Bill: If , after makes a throw-in ko at a or b. White can also make seki at a.

This is not as good for White as the previous Continuation diagram.

Alternative: clamp  

b) Against clamp, is the only defense leading to a throw-in ko, with White taking first.

Variation  

Natural looking is a mistake. After , Black can't play at a! This line depends on outside situations; Black may escape by playing at any of b.

White's option  

connect underneath leads to another ko. Black takes the ko first.

c) For White's hane (c in the summary), see Shikatsu Myoki Tsumego Series 123 and Shikatsu Myoki Tsumego Series 124.

These alternatives, b) and c), are important because White can choose the direction of the resulting shape; in other words, which side (left or upper) is more affected by the ko fight. On the other hand, a) 2-2 attack above allows Black to choose.

d) For 2-1 attack trick play, see Carpenter's Square - Wrong Attack.

One outside liberty

One outside liberty  

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

One outside liberty  

in the previous diagram is forced: here's how Black dies by playing atari.

The basic method fails  

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

Continuation  

However Black squeezes White for two eyes with after White first captures the ko. The extra liberty at a tells.

See also

• Weak carpenter's square
• Carpenter's Square - Wrong Attack
• Shapes related to the carpenter's square
• Joseki-related life-and-death example 8

Book references

• Get Strong at Life and Death Part 1 Section 9
• Life and Death Chapter 29 and 30
• Think Like a Pro Pae page 103ff

Online references

• All you need to know about Carpenter's Square

It seems that one as white, one would only need to learn the method that works with 2 outside liberties, as it will also work in 1 and 0 outside liberties. As black, the other variations I suppose have some value in knowing why they don't work (or work as well for white) in the 1 and 2 liberty versions... Or have I missed something?

(Sebastian:) The difference is that the edges have to be better protected. In diagram "Two outside liberties", you need the marked stone . With 0 liberties, you can do without.

[1] A wooden box used traditionally in Japan to measure one gō unit of volume, which is almost exactly 180 ml. Ichigou masu translates simply to "one gō measuring box".