All you need to know about Carpenter's Square - 30
This is one of the simplest problems in the series if white knows where to start.
is a famous tesuji, simple and effective.
is incorrect in this shape. A ko is inevitable.
doesn't work either. is important. The result is again a ko.
Conclusion: Remember the tesuji in Diagram 30.2 and it can be used in other situations too.