Carpenter's Square

All you need to know about Carpenter's Square-27

Diagram 27.1: White to play

Can secure this corner?

Diagram 27.2: White fails

appears to be important, but can only start a ko here.

Diagram 27.3: White fails

has the option to fight an alternative ko. This way white takes the ko first which is a slight improvement.

Diagram 27.4: Solution

Making / exchange first is the correct order. Then white can come back to the key point and kill the corner - black cannot play 'a'.

Conclusion: Diagram 27.4 is a simple and elegant solution.