Forum for 33 Point Invasion

Why cover the cut? [#2333]

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Sebastian: Why cover the cut? (2010-07-24 22:15) [#7833]

As shown in diagram "the joseki continues" (reproduce to the left)[thumbnail diagram], the joseki ends with B4. That is the case in all professional games that come with SmartGo, except for one: Yamabe Toshiro - Kobayashi Koichi of 1977 (s1977-10-15ca). In that game, Black (Kobayashi) did not cover the cut until much later (B65). It seems the only adverse effect for Black was that White got W60 in sente, which seems not that big to explain why nobody else ever tenukis.

black+triangle marks tenuki from joseki (instead of B4 in the article). Intermittent moves are marked black+square and white+square.  
X Re: Why cover the cut? (2010-09-07 21:12) [#7982]

Astone (on KGS) explained to me that the black stones in the vicinity work similarly to a net; I think he called it "a net before the move". We tried something similar in a game later ([ext] I didn't manage to catch his stones, but at least it helped me build nice territory on either side.

reply Answer (2010-09-08 02:22) [#7983]

There are numerous reasons to defend the cut:

  1. It connects your stones, connecting stones is good
  2. It makes good shape (eye potential)
  3. It is thick (you can push opponent against your wall)
  4. Good for endgame
  5. Good aji, honte

About the only bad thing is it is gote. It can often be tempting to tenuki, and sometimes that is a good idea. Something to note from your pro game search is if it would be good for black to tenuki, white would probably not have played that way.

Looking at these in a bit more detail:

1) Connecting

If you don't defend, white can cut and you cannot immediately capture the stone so a fight will develop. Note the tesuji of 3.

The cut  

What happens next will depend on the surrounding positions. Even if white does end up sacrificing the original 1 stone black will crawl on the 2nd line a bit so white will get stones in this area in sente and thus be able to use them to sabaki and reduce black's potential here.

I note that in the pro game example you posted black already had many stones in this area so black judged he could handle the fight.

The other cut

No other cut, part 1  

The other cut in this position is also fixed by defending the 3rd line cut with a hanging connection .

No other cut, part 2  

Yes other cut, part 1  

However, if black doesn't defend, then white could play something like this. (Note if black has the ladder to the lower right he can just capture in a ladder with 4 at b, but nets are better than ladders as they are local and don't give a ladder breaker). If black pulls out at a white may extend, black crawls on the 2nd line a bit and maybe white can hane and kill. Or white can answer a at c, if black answers at d then white can e to capture. If black omits d to save his 2nd line stones then white can escape at b which cuts black and a fight develops, the circled black stones are now cut and weak.

2) Eyeshape


a Is almost certainly an eye, b can easily become one.

3) Thickness

Attacking with thickness  

There are many ways the wall could be used for attacking, in this diagram white has invaded and black can pincer at B2 to push white into his wall. If he hadn't made the hanging connection then after the pincer at B2 white could cut at a and now the fights from the previous diagrams are really easy for white with the support of W1. So if black answers W1 to make the hanging connection, white has sente to extend and make a base at b, which also threatens to slide to c.

4) Endgame

Defend: white gote endgame  

Even ignoring all the cuts, the handing connection is good for endgame (and better than the solid connection, it also makes better eyeshape than the solid) as it makes white's hane connect there gote.

Tenuki: white sente endgame  

If you tenuki white gets this endgame in sente and you have 2 fewer points.

Tenuki: clamp  

In fact white has something rather better than the 1st line hane in the last diagram, though it can be gote: the clamp tesuji of 1. Black can't descend to a or white cuts at b and either the circled on squared stones are captured.

Tenuki: clamp, variation 1  

Black can play this way and take sente, but he has lost many points on the top side. a is a big point, black may actually exchange it for b before tenuki because if white plays there he then has to start worrying about eyes for his wall.

Tenuki: clamp, variation 2  

In this variation black plays 4 to keep some territory on the top. However, he takes gote to do so which means white's clamp reduced in sente which is good for white. Here white can either connect from the atari of B6 in sente and black defends the cut of a, or white may cut at a and fight a big ko.

5) Honte

Playing honte to not leave bad aji is one of those things that takes time to appreciate. The aji of the cut gives white also sorts of tactics, here is one idea:

Bad aji  

Black is trying to make some points at the top and omit defending the cut. White contacts at W1. Now if black stands at b the white cut at a is severe as W1 will be useful in the fight. If black answers W1 by defending the cut, white gets to play 2 moves in a row on the side stone with b.

Aji in real game

Artem92 8d (White) vs duelist 6d (Black) on KGS  

Here is an example from a real game, [ext], of the bad aji from leaving the cut. Though in this game black was right to not defend the cut as it would be overconcentrated and sente was very important. White would like to enter the right side to stop black getting too much there. Crawling on the 2nd line at a is not so good, so artem uses the cut aji.


White has now entered the right side, alive as connected to the corner, on the 3rd line, in sente! However, I think black made a mistake and 2 should be at 3 to avoid the squeeze.

X Re: Answer (2010-09-09 07:23) [#7987]

Thank you very much for this extensive discussion - I tossed out a brick and received a jade! I'm especially amazed that you found a real high dan example for this. In that game, it looked like Black had plenty of stones around to provide the "preexisting net", and still they weren't enough to completely neutralize the bad aji.

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