All you need to know about Carpenter's Square - 21

Diagram 21.1: Black to play

Let us discuss this corner shape seemingly weakened by White square . Suppose white just plays White cross , how can black respond?

Diagram 21.2: White fails

Black 1 is incorrect, but White 2 is wrong too. Black 3 is the key point and black is alive.

Diagram 21.3: Black fails

White 2 occupies the key point instead. Black 3 is the most tricky answer (clearly Black 7 or White 8 not working). Although Black 9 is a good move (a common tesuji to avoid those big-eye killing shapes), it doesn't work here......

Diagram 21.3: Black fails (continued)

White 12 is a calm move. Black is killed.

Diagram 21.4: Solution

It becomes clear now that Black 1 is the key point. If white plays White 2 , black has a number of forcing moves, followed by Black 9 .

Diagram 21.5: Solution (continued)

White 10 can be a tesuji sometimes. But because of the external liberty at 'a', black can use Oshi-tsubushi tesuji to kill the white stones.

Diagram 21.6: Black fails

Black 1 also looks like a key point, but so is White 2 . If black chooses Black 5 , white can form almost a picnic ko. Black clearly fails.

Diagram 21.7: Black fails too

Black 5 here is better. After White 10 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.

Diagram 21.8: White fails

Black 3 is slightly more challenging. White 4 is incorrect. Black can play Black 5 and the corner is completely alive.

Diagram 21.9: Same as Diagram 21.6

Apparently White 4 is the key point. This results the same as Diagram 21.6.

Diagram 21.10: Black fails

Black 5 doesn't work and the corner is killed by White 6 .

Conclusion: Diagram 21.4 is the only right solution. Luckily it is not hard to memorise that: Black 1 is almost always the key point for Carpenter's Square shapes.