I've been wanting to interject in this discussion for a while, and now that Ed's reached his conclusion it seems reasonable to do so. And my point is that in fact George's initial move 2 was actually better than Ed's suggested move 2! This is a rather more subtle point than George's initial mistake of thinking there was a problem in the corner that necessitated white 18, so is aimed more at Ed and sdk or low-dan level players (though hopefully George can follow), indeed it is a shape mistake I still sometimes make at 4 dan (because of the same instinct that Ed has of atari from 2nd line being normal).
EdLee wrote: FuriousGeorge wrote:
- Click Here To Show Diagram Code
[go]$$B post 22 var 1
$$ . . . . . . . . |
$$ . . . X X O . . |
$$ . . X . O O . . |
$$ . . X O 1 4 . . |
$$ . . 5 3 2 . . . |
$$ -----------------[/go]
 is not a cut; it's a clamp. And your  is incorrect; can you find the correct local sequence ? ( In your var,  is incorrect, and  is also incorrect. )
EdLee wrote: FuriousGeorge wrote:
- Click Here To Show Diagram Code
[go]$$B Take 4
$$ . . . . . . . . |
$$ . . x X X O . . |
$$ . . X . O O . . |
$$ . . X O 1 2 . . |
$$ . . . . a . . . |
$$ -----------------[/go]
In my original variation I atari from underneath, allowing Black to run in. If I atari from the right, even if Black tries to save at A, there's nowhere to go. Thank you for the patience; I was looking for this atari in take 4, but because of other local properties (say, (x)), this wasn't so clear.
To simplify the position I will add back on the 2 lines above (to make it clear the white group is totally alive), and get rid of that potential cut in the wall:
- Click Here To Show Diagram Code
[go]$$B White to play
$$ . . . . . . . . |
$$ . . . X . O . . |
$$ . . . . . . . . |
$$ . . X X X O . . |
$$ . . X . O O . . |
$$ . . X O 1 . . . |
$$ . . . . . . . . |
$$ -----------------[/go]
Black's wedging in move of 1 (I wouldn't call it a clamp, maybe an atekomi) isnt't a good move, just giving white a dead stone, but how best to answer (locally)? The problem with Ed's atari from the right (the usual instinct to capture a stone on the 2nd line) is that black's first line descent becomes sente to save the stone (would probably be played much later). This can have quite some endgame value and in other situations can even affect the life and death status of the black group (I have once made this mistake of atari-ing from the wrong side which meant I was unable to kill the black group where I would have otherwise been able to):
- Click Here To Show Diagram Code
[go]$$B 2nd line atari, then descent.
$$ . . . . . . . . |
$$ . . . X . O . . |
$$ . . . . . . . . |
$$ . . X X X O . . |
$$ . . X . O O . . |
$$ . . X O 1 2 . . |
$$ . . 3 . . . . . |
$$ -----------------[/go]
If white captures the stone then that's a sente endgame gain for black (especially if white's 1st line hane connect there was sente), and if white ignores (more likely to be best) then later black can save one stone as white is short of liberties to cut at a:
- Click Here To Show Diagram Code
[go]$$B 2nd line, then descent, later save stone.
$$ . . . . . . . . |
$$ . . . X . O . . |
$$ . . . . . . . . |
$$ . . X X X O . . |
$$ . . X . O O . . |
$$ . . X O 1 2 . . |
$$ . . 3 a 5 . . . |
$$ -----------------[/go]
However, if white plays atari from the 1st line, then black's descent to 3 has a much smaller follow-up (he can't save 1, only play some ataris forcing white to capture and fill in at 1):
- Click Here To Show Diagram Code
[go]$$B 1st line atari, then descent.
$$ . . . . . . . . |
$$ . . . X . O . . |
$$ . . . . . . . . |
$$ . . X X X O . . |
$$ . . X . O O . . |
$$ . . X O 1 . . . |
$$ . . 3 . 2 . . . |
$$ -----------------[/go]
And if black doesn't play the descent, the exchange of 1 for 2 is actually negative, because it means when white plays the first line hane white has a choice of cutting at a for a ko as well as simple connect at b if black blocks (though white stands to lose a lot from the ko as well as black). And if black pulls back at a instead of 5 because he is scared of the ko that's a 2 point gain for white (compared to if black hadn't played his silly wedging move of 1, and white just did the normal endgame 1st line hane, black blocked as no ko possibility, white connects and black connects).
- Click Here To Show Diagram Code
[go]$$B 1st line atari, later endgame.
$$ . . . . . . . . |
$$ . . . X . O . . |
$$ . . . . . . . . |
$$ . . X X X O . . |
$$ . . X . O O . . |
$$ . a X O 1 . . . |
$$ . 5 4 b 2 . . . |
$$ -----------------[/go]
So the 1st line atari has advantages if black plays next in the area (descent not sente to save stone), and if white plays next (possiblity of (fairly heavy for both) ko if black blocks). If black is building territory on the lower side and white's 1st line hane is sente then probably it will be white who plays first, and probably white doesn't want to fight the ko so it ends up not making a difference whether you played 2nd or 1st line atari, but there is an important difference between them which in other situations can be the difference between life and death, or quite a few endgame points, so it's worth learning the lessons from this shape.
P.S. There is a downside of the 1st line atari, namely the peep at 2-2, but that's not a problem here (and one reason I showed the white group extending up the side).
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R4 is quite big.
Bad. The lower right corner is already W's --
The game move extend D17 is good;
You need to reply to 
. W is happy to extend to D13.
No. B hanes at D13, the local shared vital point.
Connect. Back in post 1:
In your variation for 
This time you connected (correct). Re: 
E11.
, and stopped -- Re: posts 4 & 5. 
, that if he takes the stone, I can take back and break the ko from underneath. I had a game where there was a sort of ko cascade in several connected diamonds where I was able to flip a corner I was trying to repeat my success. I remember thinking something like this...
from P6 to Q6.
If you are interested in more, Justin Teng has a nice review books in his