Why is good shape bad?

[Kirby]

$$W
$$------------------------
$$| . . . . . . . . . . .
$$| . . . . . . . . . . .
$$| . . . X O O . a . b .
$$| . . . X . X O . . . .
$$| . . . . . X c . . . .
$$| . . . . . . . . . . .

Click Here To Show Diagram Code

[go]$$W
$$------------------------
$$| . . . . . . . . . . .
$$| . . . . . . . . . . .
$$| . . . X O O . a . b .
$$| . . . X . X O . . . .
$$| . . . . . X c . . . .
$$| . . . . . . . . . . .[/go]

In this corner pattern, the known move is B, not A. The reason is also efficiency.

Did you actually check with Katago? In my games (15-block) Katago seems to like A about as much as B but often prefers C. I know that A looks slow but it doesn't leave any weaknesses so seems fine.

I remember a similar pattern on some BadukTV commentary. IIRC, the guy was saying that 'b' was efficient in terms of territory. But stronger players sometimes start to play a little closer (one to the left of b), and even to the point of playing 'a'.

Again, IIRC, he was saying that a move like 'a' offers better opportunity for counter attack to the shape. So it's a tradeoff between having efficiency in surrounding territory on top vs. losing some of that efficiency to make a stronger attack on the left later.

I'm sorry, it seems I mixed up the names Kirby and Knotwilg!

Sounds about right. Knotwilg is more creative than me with that kind of stuff. Though, we do both have "K" names.

[CDavis7M]

I was just reviewing a game from 1851 and I thought that a joseki looked familiar and so here I am.

In this game, Black extended at 29 instead of playing the tiger's mouth at 'b.' The Commentator does not discuss 'b' nor the Kaizen sequence. Instead, he states that 25 (preventing White's press) for 26 is now [decades ago] considered unfavorable for Black. Black should just play 'a' instead of 25. Of course in this position Black 'a' is an extension from a wall.

$$Wcm24
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X O . . . . . . . . . . . . . . |
$$ | . . X O . O . . O . . . . . X . . . . |
$$ | . . O X O . . . . , . . . . . , . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . b . . . . . . . . . . . . O . . |
$$ | . . . . 4 6 . . . . . . . . . X . . . |
$$ | . . . 2 3 5 . . . . . . . . . X O . . |
$$ | . . X . . . . . . . . . . . . X O . . |
$$ | . . . , 1 . . . . , . . . . . , O . . |
$$ | . . . O . . . . . a . . . . X . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------

Click Here To Show Diagram Code

[go]$$Wcm24
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X O . . . . . . . . . . . . . . |
$$ | . . X O . O . . O . . . . . X . . . . |
$$ | . . O X O . . . . , . . . . . , . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . b . . . . . . . . . . . . O . . |
$$ | . . . . 4 6 . . . . . . . . . X . . . |
$$ | . . . 2 3 5 . . . . . . . . . X O . . |
$$ | . . X . . . . . . . . . . . . X O . . |
$$ | . . . , 1 . . . . , . . . . . , O . . |
$$ | . . . O . . . . . a . . . . X . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

The commentator states that when black pushes at 29, the continuation to 37 is natural. So there's that. The commentator does not mention the different variation mentioned by Takagi Shoichi.
To speak more about this joseki, the Commentator mentions that with the ko, Black cannot ignore White 'a' otherwise White will kill with White 'b,' Black 'c,' White 'd.'

$$Wcm30
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X O . . . . . . . . . . . . . . |
$$ | . 9 X . 0 O . . O . . . . . X . . . . |
$$ | . . O X O . . . . , . . . . . , . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . 3 1 X X . . . . . . . . . X . . . |
$$ | . a . X O O . . . . . . . . . X O . . |
$$ | . d X . . . . . . . . . . . . X O . . |
$$ | . 8 . 4 O . . . . , . . . . . , O . . |
$$ | . . 2 O 7 . . . . . . . . . X . . . . |
$$ | . b c 6 5 . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------

Click Here To Show Diagram Code

[go]$$Wcm30
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X O . . . . . . . . . . . . . . |
$$ | . 9 X . 0 O . . O . . . . . X . . . . |
$$ | . . O X O . . . . , . . . . . , . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . X . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . O . . |
$$ | . . 3 1 X X . . . . . . . . . X . . . |
$$ | . a . X O O . . . . . . . . . X O . . |
$$ | . d X . . . . . . . . . . . . X O . . |
$$ | . 8 . 4 O . . . . , . . . . . , O . . |
$$ | . . 2 O 7 . . . . . . . . . X . . . . |
$$ | . b c 6 5 . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]

This is Game One of the Nijubanjo between (W) Shusaku 6-dan and (B) Sekiyama Sendaiu. Black ended up resigning. The commentary is from Invincible and was based on a commentary by Ishida Yoshio.

So, it seems like the pushing move is OK. My version of KataGo says that the Tiger's Mouth loses over 2 points. Still, the tiger's mouth seems like a safer move. So I got my take-away.