Don't use thickness to make territory
This is one aspect of the proverb play away from thickness.
The advice is not to play short extensions such as here, which gains only a handful of points. Black's position to the right is very thick - it can only give its full strategic value if Black uses it in a dynamic, fighting plan.
Black should play from the other side, with here. Then looks like a mistake: to attack is good.
In any case the territory on the upper side is important mostly as a base for White's group, not for the few points it represents.
The fundamental ideas here for Black's use of thickness are thickness should be used for fighting and Black will take profit elsewhere, in this case in the left corner.
Another useful way to analyse is to say that Black already has some points such as the circled ones because of his outside strength here. A black play at a is overconcentrated because it is only claiming points to which Black already has a right.
Of course it is acceptable to make territory with thickness if
- it is a big territory; or
- it is the endgame.
What one should not do is to make small, unambitious territories based on thick positions early in the game.
MK So white should start at ? Then Black goes at and makes a perfect extension from three stones wall. Is a mistake?
Bill: The rule of thumb about extending from a wall does not apply to a wall with eye space, only to a wall that needs an extension. Black's corner needs a shimari more than his wall needs an extension. allows Black to make a shimari.
Perhaps Black's ideal play is , combining shimari and extension. The 5 space extension is good, as it does not leave White's invasion, , room for a two space extension after .
As for White's play, I think that each of a - e is worth considering.
Charles Here White c can be considered normal, to seek quick life. If White d, the intention would be to play the nadare (which would go into a small nadare line, assuming strong players). White e is a special technique, probably dependent on some ladder(s).