All you need to know about Carpenter's Square - 21
Let us discuss this corner shape seemingly weakened by . Suppose white just plays , how can black respond?
is incorrect, but is wrong too. is the key point and black is alive.
occupies the key point instead. is the most tricky answer (clearly or not working). Although is a good move (a common tesuji to avoid those big-eye killing shapes), it doesn't work here......
is a calm move. Black is killed.
It becomes clear now that is the key point. If white plays , black has a number of forcing moves, followed by .
can be a tesuji sometimes. But because of the external liberty at 'a', black can use Oshi-tsubushi tesuji to kill the white stones.
also looks like a key point, but so is . If black chooses , white can form almost a picnic ko. Black clearly fails.
here is better. After there is just enough room for black to make two eyes. Black is successful in a pure life & death sense. But comparing to Diagram 21.4, black is almost 10 points worse off so I cannot accept this as an alternative solution.
is slightly more challenging. is incorrect. Black can play and the corner is completely alive.
Apparently is the key point. This results the same as Diagram 21.6.
doesn't work and the corner is killed by .
Conclusion: Diagram 21.4 is the only right solution. Luckily it is not hard to memorise that: is almost always the key point for Carpenter's Square shapes.