Defend territory, or attack

Black to move. 'a' or 'b'?  

It seems that in the position like this most would choose b. But the problem is not that simple always.

Scartol: Okay, I'll bite. Why not?

Hyppy: I'll take a stab at it and say that if White counters with a, then Black would have two separate groups, which can both be attacked by one white group. Furthermore, White could play a two- or three-space extension along the third line at x or y to establish her own base, ruining Black's territory along the upper side. Just my guess.

Kobayashi-Kato  

The one pro game I came up with in which this happened was this (Kobayashi Koichi-Kato Masao 1997-12-11, colours reversed); in which is played first and then . There is another stone () in the picture.

In other games White used one of the 'x' points to start on this side. In fact since Black has taken up quite a low position here, this area will not have high priority until the middle game; and then how to treat it for White is likely to depend heavily on other positions.

Slide  

Of course Black at a is another idea for . But at b seems very territorial, really.

Supposing Black plays this way, and later White plays the natural shoulderhit at a to reduce, the pro examples I have seen suggest that Black can choose between attacking or taking territory.

Extension avoids pincer  

Therefore the initial position is more likely to come about via this one, where is played before Black at a: which avoids White's pincer possibilities.