A knight on a weakened central square...

12/4/2009 – ... usually means a strong positional trump. However, sometimes this alone is not enough to win the game. Here Black was facing the question whether closing the queenside with 20...b3 21.a3 (diagram) would be beneficial for him. What do you think? A) this enhances Black's spacial superiority, but doesn't yield anything concrete; B) it is a powerful continuation, considerably increasing his positional advantage; C) it's not good, White has one worry less and will be able to successfully defend himself in the centre and on the kingside. The solution is here, but first ponder over it with a  larger version of the diagram.