The black group in this diagram is called tripod group. It is one of the smallest shapes in the corner that is alive as it stands. Black needs to remember the correct reply to two white attacks.
This is the first attack. After , Black still has a live shape in the corner.
Alain : This is true in most positions, but Black should be careful when White plays in the neighbourhood!
One should keep in mind that a white move at one of the marked points threatens locally to kill the corner. Of course, White has to be very strong on the outside if she wants to play like this.
This is the second attack.
Shaydwyrm: I think this also lives, is there a disadvantage to it?
How can Black defend against ?
--Stefan Verstraeten
This move seems to work. a and b are miai, as are c and d. - Andre Engels
SnotNose: The tripod group (in the surroundings shown) is unconditionally alive. If White a, Black b. If White c, Black d. in the previous figure was a mistake.
In this context, Black must be careful not to make the 'sente' exchange as shown in this diagram.
The reason is that the status of the corner has now changed to ko. White can set up this ko following the next diagram.
Black is now forced to win the ko if he wants to keep the territory in the corner.
Black cannot play descent at :
is the killing tesuji, making a placement and aiming at setting up a snapback at a. Black is dead.
The a-b exchange creates a shortage of liberties. After in the previous diagram, if Black hadn't exchanged a for b, he could live unconditionally.
The throw-in at is the key move. After
, if White takes two stones at a, Black will take back the white stone at a.
This variation with taking the ko gives the same result as the previous diagram - unconditionally alive.
--unkx80
We have practiced the tripod group at our club. In response to ,
is the intuitive move for many players. We were wondering why this would be "wrong". I think that under these conditions, with all that open space around, this is actually a good move, but the tripod often appears in enclosed space, so that a white attachment at a becomes dangerous. Moreover, if there were an escape route along the left side, White would have blocked at
instead of
. So,
at a is the main line.
So, Black escapes, but loses his corner territory and, what's more important, his eye potential there.
This is a position from the final game of the 2000 German Championship, between Franz-Josef Dickhut (Black) and Christoph Gerlach (White). Black wanted to do something about the upper left corner.
So he invaded at . White can play
one point higher, but then Black has a cut at
, which he can use to isolate the two white stones at the top. Either way Black creates a nice position.
From an endgame point of view, it is better for White to play like this diagram: Black will end up with two points less in the corner. However in the game White followed the previous diagram, probably because he didn't like the remaining weaknesses on the left side following this sequence.