Two stones on the third line can make an eye
After , there is an eye along the edge.
Due to the presence of the marked stone, can be captured by .
Note that this does not work if there is a white stone at any of the points marked a, because then white can play at b.
Other nearby stones can also prevent the eye, so black should always verify whether the eye can really be made before relying on it.
The only way to directly prevent the eye is with and . This will only work, however, if black cannot use his sente to achieve anything towards the left, e.g. with a move at a or b
Note that this also works. Often, white can choose which side to play first. This can be an important consideration, as white may be able to prevent the eye on one side but not the other.
Again, the eye can be prevented with and , but only if black cannot get an eye or escape towards the right with a move like a.
If white is not strong enough on either side to prevent the eye, black can safely hane both sides before making the eye. This is slightly better for endgame purposes.