Sub-page of KoThreatExercise3
Black at b in variation 2 (fewer ko-threats)  

KarlKnechtel: (I'm putting in some "Advanced" level concepts here, but trying to explain them at a "Beginner" level. :) )

This is rather a sort of perverse case of "There is death in the hane", I suppose. ;) Black expects to make a slight gain by pushing into White's territory along the side, gaining one point basically. White throws a surprise by making a hane as well, even though it's in atari! Actually, this is a throw-in tesuji. Black captures, but when White plays at W4, it's all over. There is no where for Black to make eyes.

So what went wrong? The mistake in B1 is that it lets White play W2 in sente. As seen on Ko threat exercise 3 where this diagram originally appeared, playing first at either W2 or W4 by Black will be enough to make two eyes. (The original discussion is about which is better.)

After the play at W4, there is one eye at the bottom, and the top eye is sealed by playing at W2 or B3. Those points are miai - since White can't play at both simultaneously, Black takes the other and lives. Note that miai are often a good source of ko threats, and indeed White would get one there.

Here's proof that B1 at W2 (the descent) makes life:

Black makes life with the descent  

If White plays W2 anywhere else, Black at W2 divides the space into two parts. This is White's first ko threat in this situation.

B3 still divides the space into two parts, but White isn't done yet because of how the stones are positioned inside the bottom eye. W4 is the second ko threat, threatening to capture. Black must capture first, and is now safe. Of course, White would only play W2 and W4 as ko threats when an actual ko comes up; otherwise it's just wasting those threats.

Back to the original discussion. When Black plays B1 in the original diagram, White can play W2 and Black's safety is threatened - the player weak enough to play B1 probably doesn't even realize that at the moment, but is simply happy to capture a stone that White seems to be throwing away. Black is still doomed if he doesn't capture, though. For example, if he suddenly realizes the problem and plays at W4 in the original diagram to start making eyes, White plays at B3 and now that eye is destroyed - another miai situation. Once Black captures, he would like to play at W4, making two eyes. But it's White's turn, and W4 kills the group.

Basically, Black got too greedy here. Originally either W2 or W4 would let Black live - but only if he played one of them right away. So those points weren't miai.

Now, I will work my mid-kyu magic, and remove four stones from the board without changing the situation!

Another variation - correct way to penalize Black's greed  

Here the same sequence of moves is played with the same result, despite the removed stones. W4 still kills, because B3 has reduced the space to bulky five and White, having the move, strikes at the vital point. (See life and death if you don't understand this.) Again, B1 at W2 would have resulted in life, as would W4 or even a, if I'm thinking clearly.

The reason I removed the circled stones is so that the play at W2 still works; without sufficient outside liberties, the rectangular six in the corner in the corner would die. (I think I have it right that it will die with only the one outside liberty at B1...) Perhaps that might motivate a move such as B1, but you have to realize that it doesn't really extend the eyespace anyway. The reason Black had enough space to live inside the rectangular six in the corner in the original diagram is, somewhat paradoxically, because of the stone at a (making it easier to divide the space) - though Black did need one liberty at B1. (Otherwise he cannot play B3 in the second diagram because of shortage of liberties.)

Lessons learned:

  1. Don't be greedy. Ensuring life is more important than making one-point plays that only work if the group lives.
  2. In a tsumego, the person who makes the first capture often loses.
  3. Learn the throw-in tesuji. It can cause snapbacks, set up oiotoshi, and - perhaps most importantly - steal eyes. Capturing isn't everything. Again, don't be greedy.
  4. There is death in the hane. There is suicide in the hane, too, because at the edge of the board, what is hane for one player is descent for the other - that is, making a hane doesn't prevent your opponent from making one.
  5. Learn to recognize life and death situations. One eye is worth more than a liberty perhaps (see Eyes Win Semeais); a big eye is worth several liberties depending on its size (see nakade liberties); but two eyes is worth infinite liberties. Plus they count as points at the end of the game; unsurrounded liberties don't.
  6. But reread the rectangular six in the corner Dieter

KoThreatExercise3/Failure last edited by on April 15, 2005 - 17:52
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