Diagonal attachment joseki
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The diagonal attachment joseki is commonly played from a stone on the 3-4 point or 4-4 point against a low-approach stone on the 5-3 or 6-3 point respectively. It is mainly used when there is a blocking stone in place - either when White has ignored a Black pincer or White has invaded a Black extension from a corner. In modern Go the diagonal attachment joseki has many applications and appears frequently at both the professional and amateur levels.
Fundamentally the diagonal attachment joseki is always played to make the White approach stone heavy and subject the resulting two-stone pillar to attack by Black. The intensity of the attack depends greatly upon the surrounding circumstances:
- Where is the pincer stone? If it is close to White, the threat is immediate but Black may only be able to threaten to capture on a small scale. White may be able to afford to give up the stones in exchange for benefit elsewhere on the board. If the pincer is too far from White, the attempt to make White heavy may not succeed and White may settle the stones without ever coming under a significant attack.
- What lies beyond the pincer stone? If the area beyond the pincer is Black, then Black is able to adapt a strategy of building influence/territory while chasing White. If the area beyond the pincer is White, however, Black may come under attack in turn so that White and Black are left to engage in a running fight out into the center of the board.
- What lies down the left side? If the left is an area of Black influence, Black has the opportunity to turns this into territory while chasing White into the center. If the left is an area of White influence, then Black may be able to erase that influence during the attack instead.
These different matters determine the basic shape of the fight that will evolve from the original diagonal attachment. Will Black emphasize the left side or the right side while attacking White? Will White attempt to make life on the top or escape into the center? Does Black want to prevent life at the top by undercutting White or encourage life at the top by blocking White's escape? The circumstances of each game will differ and therefore the strategies of White and Black will differ as well.
Article on kosumi-tsuke on http://361points.com/articles/18/1 by Sorin Gherman, ex-insei, AGA 8 dan