(;GM[1]FF[4]CA[UTF-8]AP[CGoban:3]ST[2]
RU[Japanese]SZ[19]KM[0.00]
PW[White]PB[Black]
;B[qd]
;W[dp]
;B[pp]
;W[dc]
;B[oc]
;W[de]
;B[cj]
;W[ch]
;B[cn]
;W[gq]C[This white move used to be a popular Joseki around 2002.  It was a favorite of Yu Changhyeok 9P (winner of the second ING cup), and around this time many other players imitated it.  However professional study found a flaw in the variation, and therefore it is no longer seen so often in professional play.  Do you know why?

This was analysis was part of a game review shown on Baduk TV.]
;B[bp]
;W[cl]
;B[en]LB[dk:B][cq:A]TR[cl]C[Although possible, A is a bit insipid for white now.  Black can surround the triangle stone with B and be satisfied.  So the idea for white is to play B themselves now.]
;W[dk]LB[cm:A][dq:B][eq:D][gq:C]C[The joseki at the time was A ( see the first variation).  Playing black B used to appear in professional play, but by 2012 it had been decided that the B variation was not good for black; black is left with a serious defect.  See the second variation for the continuation after B and the problem black is left with.  However black has a bettter move than B, can you find it?  Follow the main line for the analysis of why it is better for black than B.  This new variation is why A is no longer Joseki and why  white C  rairly shows up in current professional games.

It will probably occur to you to wonder why black cannot play D direcly now, without an exchange at A.  The answer will be  white can block at B and fight.  Analyzing  this is left as an exercise for the reader.]
(;B[fp]
;W[fq]
;B[dq]
;W[eq]
;B[cq]
;W[jq]LB[fp:B][dq:A]C[If you compare this sequence following  B to a direct play at A it should be clear black has gained.  There is no Ko aji, and black can even cut and capture a stone later.])
(;B[cm]LB[bk:B][bl:D][cq:C][eq:A]TR[cm]C[At the time, joseki was for black to bump with triangle here.  If white answers with B black has a very severe move at A ( see variation).  So white would protect the corner at C, and black would connect under with D.]
(;W[cq]
;B[bl]LB[dk:A]TR[bl][cm]C[This was joseki at the time.  White has gained by forcing black to connect under with the triangle stones, instead of capturing on a large scale with A.])
(;W[bk]
;B[eq]
;W[ep]
;B[fp]
;W[fq]
;B[dq]
;W[cq]
;B[cr]LB[oc:B][pd:D][qd:C][fo:A]C[Since the ladder at A does not work for white they are in big trouble.  When you study this does D block the ladder?  How about if black only has the B stone?  ]))
(;B[dq]
;W[eq]
;B[cq]
;W[eo]
;B[fn]
;W[jq]C[Black must tenuki from the lower right now.  Can you see what the problem left behind is?  Follow the continuation to see.  wh]
;B[jc]
;W[co]
;B[bo]
;W[dn]
;B[dm]
;W[cm]LB[dq:A]C[Black has much more to lose in this ko than white.  The fact white is left with this option makes the original black move at A not so appealing.  Black will be worrying about this the rest of the game]))
