I've been reading about some of the basic double approach joseki, and there is something that I do not understand about the one below. It ends in the following position after black plays the marked stone. Why is it not joseki for white to extend from the downward facing wall, say to around the triangle at C8? Is white thick enough that the extension is inefficient? It doesn't seem thick, but there has to be some reason that white doesn't need to spend a move to ensure a base.

Click Here To Show Diagram Code

[go]$$c
$$ +-------------------------
$$ | . . . . . . . . . . . .
$$ | . . . . B . . . . . . .
$$ | . . . X O O . . O . . .
$$ | . X . X . X O . . , . .
$$ | . O X . . X . . . . . .
$$ | . O O X . . X . . . . .
$$ | . . . O . O . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . , . . . . . , . .
$$ | . . . . . . . . . . . .
$$ | . . T . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .[/go]

What does the lower left corner look like in the game when that joseki is chosen? Perhaps White already has a stone down there?

This isn't as flippant as perhaps it sounds - there's a good section in Yilun Yang's fundamental principles about two players in a contrived game where both sides pick "joseki" for each corner, and one ends up miles behind out of the opening.

Some josekis only exist because of ladders to other sides of the board - I don't think anyone would deny that the left side of the board in your diagram is very large to play in, but if a settling move is not part of the joseki, perhaps this is a joseki to pick only when ending in sente is important. As a regular double approach player, this isn't a joseki I find played very often. If you feel worried about the left hand side, perhaps this joseki instead?

Click Here To Show Diagram Code

[go]$$c
$$ +-------------------------
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . X O O . . . . . .
$$ | . . . X . X O . . , . .
$$ | . . . . . X . . . . . .
$$ | . . O . . . . . . . . .
$$ | . . . W . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . , . . . . . , . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .[/go]

The really deep questions in joseki are far over my paygrade, but it feels to me that if you're trying to treat this full sequence as an "isolated corner with no bearing on the top and left side", then perhaps you're hoping to get something optimistically conclusive out of the study?

EDIT: Looking further into some of the interesting lines on this, I came across this from a 20th century title match pro game:

Click Here To Show Diagram Code

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

(Black followed with "a", White with "b")

I suspect there are a huge number of ways this corner can be expected play out with respect to the top and left hand side, and whether it's "good for White" will depend on a lot of what else is going on there

Good to see you back on the forum, topazg

_________________
be immersed

Click Here To Show Diagram Code

[go]$$Wc One joseki
$$ +-------------------------
$$ | . . . . . . . . . . . .
$$ | . . . . X . . . . . . .
$$ | . . . X O O . . O . . .
$$ | . X . X . X O . . , . .
$$ | . O X . . X . . . . . .
$$ | . O O X . . X . . . . .
$$ | . . . O . O . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . W , . . . . . , . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .
$$ | . . . . . . . . . . . .[/go]

This is one joseki, right?

But tenuki is OK, too, right?

I don't know what LZ says, but this has a handicap go feel to me, where White plays on both sides with sente, leaving some thinness behind.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

The joseki I posted is the continuation of this joseki. There is a description of it here: https://www.youtube.com/watch?v=n1A0hskZkEg. I've never had an opponent play it out this far. I also understand that the rest of the board matters. The point of my question is just that I don't think I understand what white is getting locally from the downward facing wall.

I haven't seen that last white move in my references. But the thing I don't quite follow is why tenuki is an option? Black's group is alive in the corner. So, a black stone at C10 would be sente, no? Or maybe the point is just that at this point in the joseki, the continuation depends critically on the surrounding board?

True earlier too, I think.

_________________
The Adkins Principle:
At some point, doesn't thinking have to go on?
— Winona Adkins

Visualize whirled peas.

Everything with love. Stay safe.

I've played this joseki and variations thereof in a few of my British championship games, here's a video lecture I did on one vs Andrew Kay:
https://www.youtube.com/watch?v=wI8Sdsie2QQ
And this year vs Sam Aitken some text LZ analysis:
viewtopic.php?p=234158#p234158