All you need to know about Carpenter's Square - 20

Diagram 20.1: White to play

Does help in any way?

Diagram 20.2: False hope

is incorrect. Black is expecting this sequence similar to Diagram 19.2. would help in this case. However......

Diagram 20.3: Black fails

is the correct answer. Note that white can use this sequence in Shape 19 as well.

Diagram 20.4: Continued (12= , 13= )

Black is one liberty too short and cannot play 'a'.

Diagram 20.5: Black fails

So black has to block at . However, is incorrect. This result is the same as Diagram 19.3 and black fails to make use of .

Diagram 20.6: Solution 1

Unlike Diagram 19.4, tesuji now works. Thanks to black can throw in to start a ko fight.

Diagram 20.7: Variation

may atari from the other side, forming a different ko. But note that black takes the ko first so this result cannot be good for white.

Diagram 20.8: Solution 2

Since in the previous diagram appears to be the key point, wants to occupy that location first. To avoid the failure in Diagram 20.5, is a must. is a nice idea. Black has to go back to to avoid the Bulky Five killing shape. So the result is a ko. If black wins the ko fight by connecting at , white can still play 'a' to form a seki, destroying 9 points of territory, although black also has a yose at 'b' that can get a few points back. Overall this solution seems to be slightly worse than Solution 1 for black.

Diagram 20.9: White fails

is a thoughtless move, hoping black to block at 'a' so that knowledge in Shape 7 can apply. But is good - please verify that black is unconditionally alive now.

To summarise: black corner is quite resilient because of the hane move. Diagram 20.6 and Diagram 20.8 are the right solutions, resulting in ko.