Two stones on the third line can make an eye

   

Unless the opponent is very strong along the edge, two stones on the third line can make one eye along the edge in gote:

[Diagram]
 

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

[Diagram]
 

After B3, there is an eye along the edge.


Why it works

[Diagram]
 

Due to the presence of the marked stone, W4 can be captured by B5.

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

[Diagram]
 

The only way to directly prevent the eye is with W2 and W4. 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

[Diagram]
 

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.

[Diagram]
 

Again, the eye can be prevented with W2 and W4, but only if black cannot get an eye or escape towards the right with a move like a.


Playing both sides

[Diagram]
 

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

[Diagram]
 

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



Two stones on the third line can make an eye last edited by HermanHiddema on May 30, 2014 - 12:03
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