In an attempt to understand the triggers that set off a pro's thought processes, we have been studying a variety of shapes. Among them were the kosumi (diagonal connection), tobi (one-space jump), the mouth shape, the bamboo joint and variations and combinations of the latter two. We also learned the empty triangle is a no-no.

Defects of kosumi

Diagram 1 :
The kosumi (the diagonal connection shown in this diagram), is a very strong, unsplittable shape in its basic form. For example, if White plays at A, Black plays at B, and vice versa (this vice versa situation is called miai). A and B are miai and therefore the black stones are always connected.

Diagram 2 :
Last week, we established that black 1, as well as A, B and C in this diagram, are vital points for attacking the kosumi because they induce damezumari (shortage of liberties).

Problem 1 :
Black has lost the two stones on the right side. They can't be saved, but they can still be used in many different ways. Study Diagram 2 and try to find the most beneficial way to sacrifice the two black stones.

Problem 2 :
Black would like to separate the two white groups. How can Black attack the group on the right most effectively ?

Problem 3 :
A common saying in go is: "My opponent's vital point is my vital poin." Where is White's vital point for attacking the black formation? If you can figure that out, you will know the answer to the problem: How should Black defend his corner ?

Problem 4 :
White has just played the marked stone. What should Black do to defend against this move ? Keep in mind that after your move, it is White's turn again. That sounds obvious, but many people are just happy to connect or to play any move that seems to achieve the immediate objective. However, when playing, Black shouldn't leave behind vulnerable points.

Problem 5 :
Black's marked stones seem to be in trouble. Where is the vital point for Black to attack White's weakness ?

Solution 1 :
This is a fairly difficult problem, but after the explanation about the vital point of the kosumi, you should be able to find the solution. Black 1 is correct. White is forced to connect at 2, after which Black forces with 3 and 5. This is the most effective way to sacrifice the two marked stones. Later on, Black can connect 1 by playing at A.

Reference 1 :
Black 1 is bad style. Even after playing 3 and 5 to sacrifice his two stones, black 1 would have been more effectively placed at B. After black 1, even when Black plays at A, White will block at B. And even if White didn't block at B, a black move at B would not be answered at 2, as was the case in Solution 1.

Solution 2 :
Black 1 makes optimal use of White's weakness. This attack forces White to play moves 2 through 6, allowing Black to completely seal White in with 7 through 11.

Reference 2 :
Black 1 is poor style as it gives White good shape. Black 1 and 3 seem to split the white positions, but in fact these moves may create a weak group if White cuts at D. Black can then play at A, which White has to answer at E (to maintain two eyes). But even if White doesn't cut at D, Black will be hard-pressed to keep White from linking up if White plays at A. Black will probably have to give way and defend at B, allowing White to connect underneath with C.

Solution 3 :
Black 1 is correct. This reduces the value of a move at A to a mere endgame move. Note that if it were White's turn, this would be where he should play, as it is the vital point of Black's kosumi connection.

Reference 3 :
Black should not block at 1. This allows White to play a nozoki (peep through move) at 2. After white 4, if Black plays at 5, preventing White from linking his stones, White easily stays alive with 6. Black loses all of his corner territory, while his stones around black 3 may even end up weak. This is a fiasco.

Solution 4 :
In response to the marked white stone, black 1 is correct. A black move at A is sometimes possible, but in this case, White would continue the attack with a move at B--remember the mouth shape and its vital points? The virtue of black 1 is that it does not leave behind any weaknesses.

Reference 4 :
Black 1 does the job of connecting, but it has no impact on either of the two marked white stones. In addition, it leaves behind a vital point at A, where White can attack later if not immediately.

Solution 5 :
Black 1 hits the vital point of White's position. If White tries to keep his stones connected, the resulting exchange to 14 is inevitable. Black 15 makes a good shape while linking up all of his stones, while White lives in a small way on the upper side. This is obviously a great success for Black.

Reference 5: Black 1 lacks sophistication and misses the vital point. The sacrifice maneuver 3 through 9 is sometimes a clever way of playing, but he ends in gote. White can now keep up the attack with A, followed by black B, white C, black D and white E. As an alternative, White can exchange B for D, then press at F (remember the vital point!). If White wants to be brutal, he can also cut at D.