Joseki-related life-and-death example 8

Joseki  

First we start with this 4-5 point 4-3 approach joseki. can be played at a, but Black can also choose to play elsewhere.

Continuation  

If Black chooses to play elsewhere, then White has the forcing move at .

Continuation  

At a later time, to seals up the left side, making it solid.

Carpenter's square  

After the previous diagram, the to sequence can be seen as sente, because it turns the black group into a carpenter's square. The placement at a will turn the corner into a ko.

The carpenter's square is extensively discussed on its own page.