Three-space extension
Compared with two-space extension
Compared with the two-space extension, the three-space extension has two advantages, and one clear disadvantage.
To start with the disadvantage. White can invade at here, and (unlike the case of the two-space extension) Black cannot force a connection by immediate means. What Black usually does is to jump out, for example at a, enlarging the scale of the struggle; or to play at b (possibly leading to a later sacrifice), to make the most of tactics locally. Therefore this length of extension isn't easily handled in all positions.
One major reason to choose to play the three-space extension can be seen in this sequence. If White is about to play the diagonal attachment at , Black's marked stone ends up on an ideal point after the proper answer
(see extension from a wall). One line closer is over-concentrated, that is, worse, from the point of view of good development.
The second possible advantage is if Black is seeking to play lightly - stay out of trouble.[1] This type of sequence is appropriate if White is strong in the upper right, as here. In that case Black a, White b might be good for White, and Black is better allowing in return for
.
Here Black can be quite happy with his development.
In some contexts completing the shape with Black at a (instead of jumping out at ) might be better.
List of basic types
There are pages dealing with invasions in these extensions on the three-space extension invasions path.
[1]
, approaching White's wall, is problematic, while extending only to a instead of
allows White to play at b.
-
press against
, preparing for a safe extension to
. This sequence avoids the problems of an immediate extension.
Charles This idea of Bill's, to use the 3-5 point low approach, press, is a bit more subtle than may seem at first glance. For example, is held back one line, giving White much less chance of fighting on this side.