Use Thickness to Attack
Black to play in Dia 1
Think about this position first without looking any further for hints, just like it would be a real game.
Black has built some thickness towards the lower side - by chasing the white group in the center - it is time now to put that thickness to good use.
The white group in the center doesn't have definite eye shape yet, but there is no way to kill it. So black has to focus on the lower side, where white is weak.
I have listed what I think are the most obvious choices for Black's next move, from A to E. Which one do you want to play?
Playing kakari from the outside - black 1 in dia 3 - is the usual approach move. There is even a proverb about it: "Approach from the wider side".
There are several problems with playing the "usual" move in this case, though:
- Black's left side is smaller than it looks: white has a big move at A in sente, and it is also open from the center at B (although B might actually be Black's sente against the white group in the center - depending on your reading abilities.)
- When white answers calmly at 2 in dia 3, he is reinforcing his lower side.
Overall, the approach in dia 3 doesn't look too inspired for black - for one thing, he didn't make use at all of his thickness.
The inside kakari in dia 4 is more in the spirit of using Black's thickness.
However, the plain replay at 2 (which otherwise isn't the best local move on the lower side) works well for white: black loses the invasion at A, so if anything he's splitting white into 2 living groups, there is nothing to attack anymore for black on the lower side.
Black 1 in dia 5 is a more lightweight way to invade the lower side, compared to dia 4.
It doesn't work well here, though: white can simply defend at 2 in dia 5, and again Black's splitting move resulted in 2 white groups that are not easy to attack: black is not using his thickness well.
Closing the lower right corner with 1 in dia 6 is the slowest of all moves so far: definitely not in the spirit of using Black's thickness for attack.
And the winner is... invading at 1 in dia 7.
Black is plunging in the middle of White's position at the point where Black's maximum influence is, in the lower right.
White can try to be smart and exchange 2 for 3 in dia 8 before invading the lower right corner with 4, but with Black's influence in the center the 2 white stones on the middle of the lower side turn to be a burden if black plays kakari with 11 next.
In dia 9 white is keeping his stones connected, but the lower side turns into black territory, and White's position still have cutting points at A and B. And White's influence is not effective because of Black's influence in the center. Black is happy.
If white connects underneath as in dia 10, he gets the side territory, but black will make about 15 points of territory in the center in sente, and can go next about reducing White's lower left corner: either the move shown in dia 10, or a 3x3 invasion would work nicely.
Dia 11 shows what happens if white directly enters in the lower right 3x3. Black trades part of the lower right side for a lot of the influence on the lower side, and it is most likely that most of the lower side will turn into black territory. This is a good exchange for black.
- The best way to use your thickness is to split and attack your opponent's stones.
- Attack and Defense by Ishida Akira