Two-Space Notchers
Two-space notchers are groups that consist of a chain on the second line, interrupted by a notch of two empty spaces on the second line and two to four stones on the third. The term was invented by James Davies. See also Notchers, One-Space Notchers and Three-Space Notchers.
The rule for two space notchers is, depending on how you want to formulate it, either "Five die, Seven live" or "Six is unsettled". For the time being we'll leave proof of the two extremes to you and demonstrate the unsettled case.
If Black goes first, he can live. White is again rather stubborn to keep trying with 2 and 4, but after 5 Black has completely constructed the two eyes he needs to live.
White can start her attack with a hane at 1.
(Reducing your opponent's eye space like this should always be the very first option you consider when examining the status of one of his groups, by the way. See also the article on approaching a life and death problem the right way.).
Even if Black were to resist, White can keep his group dead.
unkx80: See also Two Space Notchers Not Fully Connected.
Classification
Classification...
211
All of the 211xx are dead
212
All of the 212 are dead
222
All 222 are "Black can live, White can kill" except 222WW, which is dead.
213
All of 213 can make life, except for 213WW which is dead
223
All 223 are alive, except for 223WW, where Black can live and White can kill.
However, 223WSNotcher shows a killing sequence for White.