Wedge if possible
If the wedge (a) works, it will be more severe.
White at c is common and is discussed here.
White outside atari
A common continuation following the wedge of (see four die but six live). For White to choose this line of play, it is desireable, but not an absolute requirement, that there be a White stone in the upper left.
Jared: I think is the only move, but maybe could be at a.
White inside atari
Another possible continuation, often seen when the two marked Black stones are part of a moyo.
This continuation is good for black. This is why black should wedge in the first place. White's original two stones are split (with one captured). The central point: If this ladder works for black, he should wedge, aiming for this diagram.
Bill: Isn't there a better page title? Wedge if possible is not good general go advice. It is specific to this kind of invasion an response. Perhaps the thing to do is to make this a subpage?
OscarBear?: This doesn't seem to be a proverb, but rather a piece of advice about a common invasion.
Bill: Advice by whom? Anyone worth listening to?
tapir: Seems fairly common advice to be honest. See: LowInvasionOfMixedThreeSpaceExtension#toc1