is the three-space extension along the third line, like other extensions a big fuseki point which creates a base for the group and prevents White from attacking around the same point. Compared with the two-space extension, the three-space extension develops more quickly, but has the obvious defect at a where White can invade, one of the many three-space extension invasions. This is always a consideration when making the three-space extension.
It is also possible to make the three-space extension on the fourth line.
This three-space extension (J: dai-dai-geima-biraki) could also be called a very large knight's move extension; in the article three-space extension invasions it is described as a ‘mixed three-space extension’, and invasions at a, b, c and d are listed.
Here, Black makes a three-space extension from a wall.
, a one-point jump, is another alternative for developing from the three-space extension.
If for whatever reason White plays the diagonal attachment at , Black's marked stone ends up ideally placed after the stand of (see extension from a wall). One line closer at a would be over-concentrated, that is, worse, from the point of view of good development.
Depending on the situation, Black may also choose to treat one of the stones in the formation lightly for the sake of development. This type of sequence might be 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 .
Bill: In this particular situation, is problematic because it approaches White's wall, while extending only to a instead of allows White to play at b.
Bill: To avoid the problems of an immediate extension, - press against , preparing for a safe extension to .
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.