All you need to know about Carpenter's Square - 34

Diagram 34.1: White to play

The corner is much stronger with the first-line hane at Black square . Can white do anything?

Diagram 34.2: White fails

Because of Black square , the solution for Shape 33 stops working. If white continues to play 'a', black is able to use 'oshi-tsubushi' to kill white.

Diagram 34.3: White fails

If White 5 occupies this key point first, Black 10 is a good move.

Diagram 34.4: Seki

To continue from the previous diagram, the only thing white can do is to make White 11 / Black 12 exchange. Please confirm the corner is a seki.

Diagram 34.5: Can white form a ko?

Since white cannot kill the corner, an outcome of ko would be the next best. White hopes black to play 'a' so that Diagram 33.3 applies; or if black playes 'b', white can choose one of the ko options in Shape 7. However, black has better defence.

Diagram 34.6: Black alive

Black 2 is a famous tesuji! If White 3 , black occupies the second 1-2 point and there is just enough room to make two eyes.

Diagram 34.7: Black alive

If White 3 occupies another 1-2 point, Black 4 is good, recovering Diagram 34.2.

Diagram 34.8: Black alive

If White 3 , Black 4 is good. Black makes the first eye in the corner, leaving miai point 'a' and 'b' for the second eye.

Diagram 34.9: Black alive

If White 1 , Black 2 is the correct move, ensuring to recover Diagram 34.2 again.

Diagram 34.10: Black alive

If white makes White 1 / Black 2 exchange first before blocking at White 3 , Black 4 is still the tesuji, leaving 'a' and 'b' miai points to live.

Diagram 34.11: Black alive

To reply White 5 , Black 6 is still the best move. White has a number of options. Clear 'a' and 'b' are miai points.

Diagram 34.12: Black alive

To reply White 7 , Black 8 is calm, still leaving 'a' and 'b' as miai.

Diagram 34.13: Black alive

Finally, White 7 results in a double-ko. In most cases, black would be unconditionally alive.

To summarise: with the first-line hane, black corner is very safe. The best white can get is seki in Diagram 34.3 and 34.4. Black needs to know the 1-2 point tesuji. In very rare cases, white may consider Diagram 34.13 (see how a double-ko may be exploited in this pro game ).