All you need to know about Carpenter's Square - 1

Diagram 1.1: White to play

We will start building our knowledge from this rather unusual shape - stones all the way down to the border lines, probably hard to find in real games. At first sight, the corner doesn't look very secure. What can white do?

Diagram 1.2: Solution

Note the symmetry of this shape so I will only cover half the possibilities. Clearly White 1 is the only way to attack. This results in a seki. Black can omit the Black 4 / White 5 exchange and play Black 6 directly (if white then plays at Black 4 black can occupy the 1-1 point, forming a different seki). Alternatively, black can play Black 6 instead of Black 2 , followed by White 3 and Black 2 leading to the same outcome.

Diagram 1.3: Variation

White may choose to play White 3 instead. Black 4 is the best answer, also resulting in a seki. After White 5 black can feel free to play tenuki. There is nothing left white can do in this corner (you may want to verify that this would not end up in dead shapes such as Bulky Five). This is clearly in black's favor in comparison to the solutions above because black has sente now. Black 5 as a response to White 3 would be incorrect - a ko is then inevitable (either a 10000-year ko or a multi-stage ko).

Diagram 1.4: Tenuki

The corner is amazingly resilient. Even if black plays tenuki in response to the 2-2 attack, white cannot kill it. The variation shows just one possible continuation. White 3 is the key point, resulting in a ko. If White 3 plays 'a' instead, black 'b' would results in a seki - a horrible results for white, after spending two extra moves in the corner.

Conclusion: this corner is not as weak as it appears. Normally seki is the best result for both sides.