A two-space or two-point extension (two-space jump along the third line) is often used to create a base for a group, and is the minimal base required to have a hope of creating two eyes on the edge. This is one of the most common formations in go.
Because of the proximity of the edge, a two-space extension on the third line cannot be cut apart by the opponent, without other stones nearby.
After Black approaches at , White retreats to
with a two-space extension (it skips two spaces).
Extending two spaces like this isn't quite as stable as the one-point jump, but it expands further, and may create enough potential eye space for White to live.
Black cannot cut the two-point extension in this situation (should White choose to connect below).
These other two-point extensions with one stone or both on the fourth line are also useful for developing groups with a base; but they are less stable than the two-point extension on the third line, in fighting positions.
See also: