Blocking off

Simple example  

For example here is blocked (stopped as a major threat to White's territory on the right) by .

Perhaps White will feel like playing at a before , rather than be forced: that's a sound attitude. But is likely to be necessary to preserve White's area. This is rightly mentioned on the basic instinct page (point 5 there). From the point of view of strategy one may wish to exchange territories, or put more emphasis on the centre. The block is, however, a fundamental idea, for the simple reason that if White ignores , Black's next play at is very good.

Follow-up  

If White does play tenuki, we can get this position next after and the immediate block .

Now Black has two further chances. Black can cut at a, at a later time when White cannot capture the cutting stone in a ladder. Or Black can play the clamp at b, an endgame tesuji. White would lose more territory.

This diagram shows the problem with blocking too directly. After , White still has a cutting point here. At different times White could answer at b (stepping back one line in order to block more securely), or at a (play more solidly, in order to counter-attack). These are two basic strategies for successful blocking when the direct approach fails for tactical reasons.

Charles Matthews