bent four

An eye shape in the corner for which, under Japanese rules, there is a special ruling applied if the position is present at the end of the game. In the first diagram, if Black were to play at A, White would capture the single stone starting a ko, the final status of the stones depending on the theoretical outcome of a potentially complex ko fight. This calculation could be avoided by White playing at A but, since under Japanese rules only spaces are counted, this costs White one point - this hardly seems fair if White could win the ko. So to simplify counting, the Japanese ruling is that ,at the end of the game, the black stones are deemed dead without need for further play. Under the more logical standard and Chinese rules, since extra plays incur no penalty, the position is resolved by continuing play - there is no need for any special ruling.

The second diagram shows an eye shape that white can force into a bent four. Again, at the end of the game, the Japanese ruling says the black stones are dead. But standard and Chinese rules allow play to continue, in which case White can choose a seki by passing, or play for the bent four ko by filling and extending, thus forcing Black to take.