EVENT
BOARDSIZE 19
SETUP W D16 D4 O3 R3 
SETUP B C10 M3 Q4 Q16 
MARK \t@C10 A@Q10 1@O3 2@M3 3@R3
COM
Problem 6. Black to play.  

Instead of making a sanren-sei at A, Black has played the marked
stone. When White plays 3, in which direction should Black block
? 
ENDCOM
B 1 Q3
USER
CORRECT ANSWER
ENDUSER
COM
Correct Answer.

Blocking from the left side with Black 1 is the correct direction.
ENDCOM
  VAR
  B 1 R4
  COM
  Failure.

  If Black does not have a stone in the middle of the right side,
  he should not block from above ...
  ENDCOM
  W 2 Q3
  B 3 P4
  W 4 O2
  B 5 N5
  COM
  ... and play this joseki ...
  ENDCOM
  W 6 R10
  COM
  ... because White 6 eases the influence of Black's wall below.
  <=
  ENDCOM
  ENDVAR
W 2 R4
B 3 Q5
W 4 Q2
B 5 P2
W 6 R2
B 7 P3
W 8 R6
COM
The sequence to White 8 is a joseki. Even though White has taken
profit, Black is happy with his thickness. Moreover, he still
retains sente.
<=
ENDCOM