In the game of Go
), a tortoise shell is formed by the following arrangement of stones:
It cannot be stressed enough how valuable it is to make this shape, if your opponent allows it. There is a Go proverb that says "A ponnuki is worth 20 points." If this is true, a tortoise shell, being a kind of super-ponnuki, must be worth almost twice that. A toroise shell cannot be cut, has a (pretty much) guaranteed eye inside it, and radiates influence in all directions, especially in the directions of its long sides. As such, the shape is, in almost any case, alive as it stands, and will claim a significant amount of territory.
Just as a ponnuki is usually formed by capturing a single stone, a tortoise shell is usually formed by capturing a pair of stones (an iron pillar) on the two internal points. It is generally very bad to allow your opponent to do this, taking two prisoners and forming such a strong shape.