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 is the only way to attack. This results in a seki. Black can omit the / exchange and play directly (if white then plays at black can occupy the 1-1 point, forming a different seki). Alternatively, black can play instead of , followed by and leading to the same outcome.

Diagram 1.3: Variation

White may choose to play instead. is the best answer, also resulting in a seki. After 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. as a response to 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. is the key point, resulting in a ko. If plays 'a' instead, black 'b' would results in a seki - a horrible results for white, after spending two extra moves in the corner.

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