Life In 19x19

Just a little question here.
I looked up the article on good shapes.
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Both double hane and counter-hane are considered good shapes. But double hane is possible only if the previous step was counter-hane? Does that mean that this situation is good for both players? It must be better for one of them ...

thanks, got it =)

Thanks guys, I got how white can capture black by going b.

What i don't get is how black connects via ko: here white takes the stone (playing at a) and next stone disables any escape attempts by placing white stone between 5 and a.
$$W ko ...

I don't quite get it: can white capture the black group here?
The way I see it white go 1, then black 2, white 3, black 4 and the black group will connect nevertheless.
What am I missing?
What if there were no K3 stone?

Hey everyone, I am a big time newbie.
Reading the "All about life and death book" as my first.
Here is a common problem covered in the very beginning.
Can someone explain me how white "b" is good enough? Why is place stone right under "b" bad?
Thanks.