Twisted four in the corner

    Keywords: Life & Death

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

One outside liberty  

If there are fewer than two outside liberties, White can turn this position into a ko (see below). black+circle prevents that and lives with full points. (Black playing at a also does that, but leaves a ko threat, whereas black+circle leaves none.)

White starting a ko  

If B2 is tenuki, then W3 at B2 kills. After W3, White can finish the ko by filling at black+circle. If Black has no outside liberties, then white+square would be better, since filling at black+circle would leave a ko threat. After Black retakes the ko with black+circle, Black can finish the ko with black+square to live with full points. (The same applies if W3 is tenuki.)

Note that after W3, black+square would be either self atari or suicide. (The latter is if there are no outside liberties.)

2. Two or more outside liberties

Two 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.)

White cannot make a ko  

Thanks to the two circle liberties, Black can play atari at B4. Without one of those liberties, he would be putting himself into atari.

3. Endgame implications

B6 at black+circle  

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, W1 and W3 and W5 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:

Not corner twisted four  

See also:


Twisted four in the corner last edited by joegasgano on December 12, 2021 - 14:52
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