Two stones on the third line can make an eye

 

The two marked stones give black the option of making a second eye for his group.

 

After , there is an eye along the edge.

Why it works

 

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.

Preventing the eye

 

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

Symmetry

 

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.

Playing both sides

 

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.

Further countermeasures

 

is a forcing move because it makes a throw-in combination work to remove the eye.