Rectangular six
A group with a big eye with the shape of a rectangular six, is alive except for rectangular six in the corner.
Here, the eyespace is at the side, but in the center the implications are the same. The vital points are a and b and they are miai, as shown in the following diagrams.
If White tenukis, playing elsewhere, White is still alive but now lives in seki: Black can at most almost fill with a bent four. However, this seki is not valid for rectangular six in the corner.
See also