Comb Formation
The comb formation is always alive. The points a and b are miai.
The placement at is useless. Black answers at
, and subsequently 3 and 4 are miai.
When White attacks the group at , Black plays at
. Now, 3 and 4 are miai.
White here threatens a snapback at a. There is nothing to worry though, as
does the trick.
--unkx80
Shaydwyrm: A key feature of this shape is that white may take away black's outside liberties in sente. For example, in this diagram forces
or a, to prevent white from playing at
and killing with the sequence a-d.
Likewise when white takes the other liberty with , black must again respond or suffer the sequence a-c, which is double atari. These moves by white should not be played immediately, but are good ko threats - the chance to play them is unlikely to disappear, since black pushing out will be gote.
See also: