Standard opening 1
The following is a standard opening. It has been thoroughly investigated by Cho Hun Hyun and Lee Chang Ho. As a result of their experiments, it was modified substantially and often occurs in Korean pro games of the '90s. We start with four corner star points.
This joseki can not be separated from whole-board opening strategy, just like many other contemporary josekis. This means, showing the sequences of this joseki with local diagrams doesn't make sense.
was Cho 9p's new move which was different from the traditional one. In the old version, instead of , defending Black's weakness with right after had been the common sequence.
is a move which tries to make use of Black's ni-ren-sei formation on the right side. In this respect, it displays whole board thinking.
Now White faces the decision where to move next. is considered whole board joseki after which Black patches up the weakness at . is considered mandatory, because a black move there would be sente for the threat at a. Black continues utilizing his thickness to play on the left and apply pressure on White's corner. is a standard diagonal haengma to prevent being enclosed by a double keima. Probably forestalls a black keima at b.
When black approaches White's upper left corner with , white can not ignore it and instead capture the black stone on the left side . Thus, - is a very natural flow of stones, nearly a necessary order of moves.
At , the standard opening comes to an end, that is to say, professional players believe that no further research is paying off and this has found its way into standard (Korean) textbooks.
Note: there are pro games where and are played before . Also, does not seem so standard by 2007+ pro games, where White plays the more aggressive a.
White would like to resist Black's speedy play by punishing him for leaving the marked stone unattended, but Black sacrifices the stone and takes sente to play in the next diagram. This is again called active for Black. Hence is a whole board joseki mistake.
Suppose White answers the strong connection of with a reinforcement of the left side. Then is sente, that's to say, a white move elsewhere will be met with a kill at a. Surely White has got three extra: moves , and sente for , but the capture is large and without any damage on the outside. Although such a tenuki strategy would be interesting on amateur level, on professional level this is unacceptable. On the same token, allowing to be sente is not acceptable either, which is why will be played at instead.