oca wrote:
Here is an example of a move (
) in a joseki that I don't understand :
$$W what's the purpose of :b2: ?\nwhy not directly :b4: ?\nor even :b2: at :w3: ?
$$ +--------------------
$$ | . . . . . . . . . .
$$ | . . . a 2 . . . . .
$$ | . . O . O X . . . .
$$ | . O O X 1 X . 4 . ,
$$ | . . X O O 3 . . . .
$$ | . X . X . . . . . .
$$ | . . X . . . . . . .
$$ | . . X . . . . . . .
$$ | . . . . . . . . . .
$$ | . . . , . . . . . ,
- Click Here To Show Diagram Code
[go]$$W what's the purpose of :b2: ?\nwhy not directly :b4: ?\nor even :b2: at :w3: ?
$$ +--------------------
$$ | . . . . . . . . . .
$$ | . . . a 2 . . . . .
$$ | . . O . O X . . . .
$$ | . O O X 1 X . 4 . ,
$$ | . . X O O 3 . . . .
$$ | . X . X . . . . . .
$$ | . . X . . . . . . .
$$ | . . X . . . . . . .
$$ | . . . . . . . . . .
$$ | . . . , . . . . . ,[/go]
Well that sequence you showed isn't the normal joseki, normally white blocks 2 at
a, and as Sennahoj said if white doesn't but plays your 3 instead (which is another, albeit rarer, choice to avoid getting sealed in in exchange for taking a loss territorially) then black should extend at
a first in sente before jumping with 4.
So the next question is why is 2 good if white answers normally. The reason should become clear if we look at the continuation of black sealing with 4. If white now pushes out and cuts then black gets an atari at 8 in sente to make a strong wall due to the 2-3 exchange taking a liberty from white. White can then capture some stones with
b, but black has a nice strong wall (for example the cut at c can be captured with a net).
$$W what's the purpose of :b2: ?
$$ +--------------------
$$ | . . . . . . . . . .
$$ | . . . 3 2 . . . . .
$$ | . . O 9 O X b . . .
$$ | . O O X O X 7 . . ,
$$ | . . X O O 5 6 0 . .
$$ | . X . X 8 4 c . . .
$$ | . . X . . . . . . .
$$ | . . X . . . . . . .
$$ | . . . . . . . . . .
$$ | . . . , . . . . . ,
- Click Here To Show Diagram Code
[go]$$W what's the purpose of :b2: ?
$$ +--------------------
$$ | . . . . . . . . . .
$$ | . . . 3 2 . . . . .
$$ | . . O 9 O X b . . .
$$ | . O O X O X 7 . . ,
$$ | . . X O O 5 6 0 . .
$$ | . X . X 8 4 c . . .
$$ | . . X . . . . . . .
$$ | . . X . . . . . . .
$$ | . . . . . . . . . .
$$ | . . . , . . . . . ,[/go]
What if black didn't play 2?
$$W without it
$$ +--------------------
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . . O . O X 7 . . .
$$ | . O O X O X 5 . . ,
$$ | . . X O O 3 4 6 . .
$$ | . X . X a 2 b . . .
$$ | . . X . . . . . . .
$$ | . . X . . . . . . .
$$ | . . . . . . . . . .
$$ | . . . , . . . . . ,
- Click Here To Show Diagram Code
[go]$$W without it
$$ +--------------------
$$ | . . . . . . . . . .
$$ | . . . . . . . . . .
$$ | . . O . O X 7 . . .
$$ | . O O X O X 5 . . ,
$$ | . . X O O 3 4 6 . .
$$ | . X . X a 2 b . . .
$$ | . . X . . . . . . .
$$ | . . X . . . . . . .
$$ | . . . . . . . . . .
$$ | . . . , . . . . . ,[/go]
Now black has a weakness at
a/b. This result is unconditionally worse than before. The only good thing about this variation is white doesn't have the choice of ignoring the 2 move in the corner to avoid being sealed in, but that's not usually a good choice (and this seal is not such a strong seal).
In fact it could be even worse for black, white now has the choice to fight like this (next
a or
b). This would be impossible if black had the
x-y exchange because white would be in atari.
$$W fight
$$ +--------------------
$$ | . . . . . . . . . .
$$ | . . . y x . . . . .
$$ | . . O . O X . . . .
$$ | . O O X O X 0 b . ,
$$ | . . X O O 3 4 9 . .
$$ | . X . X 5 2 7 a . .
$$ | . . X . 6 8 . . . .
$$ | . . X . . . . . . .
$$ | . . . . . . . . . .
$$ | . . . , . . . . . ,
- Click Here To Show Diagram Code
[go]$$W fight
$$ +--------------------
$$ | . . . . . . . . . .
$$ | . . . y x . . . . .
$$ | . . O . O X . . . .
$$ | . O O X O X 0 b . ,
$$ | . . X O O 3 4 9 . .
$$ | . X . X 5 2 7 a . .
$$ | . . X . 6 8 . . . .
$$ | . . X . . . . . . .
$$ | . . . . . . . . . .
$$ | . . . , . . . . . ,[/go]
was a clicko, then there are five highlights covered in below commentary.
even if that is a non sense after 
) 
is the same as
in post 310, both games where played in a row, so I didn't saw your comments before playing that game.
. I still choosed to atari at
for
).
, you're pushing from behind + EdLee will come get you 
. I think the meaning of 
is that if white doesn't respond in the corner, there is a chance that white can be attacked later:
really has a lot of impact on the strength of the two groups --- black is a lot stronger compared with directly extending at 
, and white is not completely alive yet because of the weakness at the 2-2 point.
