Good Empty Triangle
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Efficiency is one aspect of shape. Late in the game, especially, it can be obviously correct to make an empty triangle and making an empty triangle can also be the vital point. A good empty triangle is an instance of what the Japanese call guzumi.
Example 1
is the vital point, even though it is an empty triangle.
makes "good shape" but becomes overconcentrated.
unkx80: I don't think it's so much about overconcentration. I believe that it's got more to do with destroying White's eye shape.
Bill: Well, it is a question of emphasis, isn't it? Anyway, let's take away the stone.
This is fine for both players, no? Black does not need an extra stone.
Example 2
This problem comes from the Tesuji Dictionary, by Segoe Kensaku and Go Seigen. They say that if you get this one right, you are 5 dan. (Of course, you have a big hint. ;-))
Example 3
The sequence of to
is one of the best ways of escaping with the lone white stone at the top.
makes the empty triangle and then White has a rather clear exit, while aiming for the cutting point at a (at the correct time).
here would be a mistake.
plays at the vital point, destroying White's eye shape, and forcing
to connect with a bad empty triangle.
Bob McGuigan: Here's an example from one of my own games (I was Black) where Black responds to a White empty triangle move by making one too. White's empty triangle looks good but after Black's response ...
in response to
is good defense.
creates a cutting point in Black's shape.
takes a liberty from White and leaves a lot of aji against White's group.