Weak carpenter's square  

The weak carpenter's square is a carpenter's square without the connection at the corner of the shape. The vital point is the same, but this time White kills the corner by playing at a.

That is, without the black stone at , changes the status of the group from "White kills by ko" to "White kills unconditionally".

If Black plays first, then playing at a lives. Black lives with a even if is occupied by a white stone.

White can kill  

After a and b are miai: a White play at a creates a squared four, while a White play at b forces black to connect at c which reduces his eyespace to a bulky five.

White can kill  

If here, then to reduces the eyespace to rectangular six in the corner.

White can kill  

There are many ways for white to kill after . This is just one variation After , at is an auto atari so Black has no other option than to let white connect at .

variation  

After a and b are miai

Endgame technique  

If Black has secured life by playing on the vital point, there is an important endgame technique for White to remember: the forcing moves at 1 and 3.

not really a carpenter's square  

What about this one? Klaus

not really a carpenter's square  

Here's one sequence. Bob McGuigan

Book references

• Life and Death Chapter 30