All you need to know about Carpenter's Square - 2
Diagram 2.1: White to play
This is what's formally known as Carpenter's Square. 'a' is clearly the key point: black can play there to secure the corner; white normally attacks there. White may also attack from 'b', 'c' or 'd'. Will discuss all possibilities later.
Diagram 2.2: Solution 1
As I will explain later, 
attaching on the first line is the only tesuji to save this corner. 
is also a good move (if attaches from one side, should always hane from the other). 
is also the only move (explained later). The final outcome is a ko fight.
There are two more things to discuss here:
- If black group has one external liberty, the above sequence remains the same, although black has an alternative to (covered in Shape 8).
- Because of symmetry, and
work exactly the same. However, in real games, the shape is unlikely to be as clean as on a textbook. So how to choose the correct side to attach? Assume that black eventually wins the ko fight, he would have captured and 
stones, leaving further yose to play on that side. For this reason, if the surrounding environment is not symmetric, black should in principle choose the side with more valuable yose left (making sense?).
Are there any variations? Let's discuss a few:
Diagram 2.3: Black fails
How about ? That would fail miserably. I will leave out a few details for readers to practice. But from white's perspective, the key is always to form an eye in the corner (creating an "Eye versus No Eye" capturing race).
Diagram 2.4: Black fails
in Solution 1 above is the only move. in this diagram is not working - the result is Bulky Five.
Diagram 2.5: White fails
in Solution 1 is also the only move. For example, in this diagram would result in seki.
Solution 1 (Diagram 2.2) is one of the two standard ways to start a ko fight in the Carpenter's Square. In Shape 3 we will discuss the second way.
