Twisted four in the corner
A twisted four which occurs in the corner may not be quite the same as the normal twisted four. The twisted four in the corner follows exactly the same logic as bent four in the corner.
|Table of contents|
1. One or no outside liberties
If there are fewer than two outside liberties, White can turn this position into a ko (see below). prevents that and lives with full points. (Black playing at a also does that, but leaves a ko threat, whereas leaves none.)
If is tenuki, then at kills. After , White can finish the ko by filling at . If Black has no outside liberties, then would be better, since filling at would leave a ko threat. After Black retakes the ko with , Black can finish the ko with to live with full points. (The same applies if is tenuki.)
Note that after , would be either self atari or suicide. (The latter is if there are no outside liberties.)
2. Two or more outside liberties
If there are at least two outside liberties, Black is alive with three points. (Three points instead of four because Black eventually needs a reinforcing move inside when White fills up one of the outside liberties.)
Thanks to the two liberties, Black can play atari at . Without one of those liberties, he would be putting himself into atari.
3. Endgame implications
When Black has exactly two outside liberties, White can fill either of them in sente. The threat is at least a ko to kill Black's group.
However, the indicated lines might be better than filling immediately, since in both indicated lines, and and each threaten to kill outright, rather than just threatening a ko to kill.
4. Not a Corner twisted four
The following is a Twisted Four and is in the corner but is not an example of what is meant by Twisted Four in the Corner. It is alive - a and b are miai: