This example shows how the carpenter's square may be formed.
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.
If Black chooses to play elsewhere, then White has the forcing move at .
At a later time, to seals up the left side, making it solid.
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.