Big P
Keywords: EndGame
The Big P is a corner shape that can be turned into a ko or a seki because of the special properties of the corner.
At first glance, Black seems to have 6 points of territory. However, after White plays a, Black can only get a seki or a ko. To prevent this, Black can turn the corner into 5 points of solid territory by playing a himself or either of the points. (Playing at a is better, though, because White could get one ko threat against the corner if Black plays at either
.)