Endgame Coupling

Coupling on the edge  

Now White is in trouble: Black will either be able to cut at a, or start a dangerous ko by cutting at b.

White's correct response  

When Black plays , White must answer this way, with .

The two white territories, on the left and right, are linked in a subtle relationship we can call 'coupling'.

White's correct response (continuation)  

After that White can defend territory easily.

In this position the coupling effect is worth two extra points for Black.

Charles Matthews