The Art of Calculation in Chess

                                                                Black to play

Here in this position its Black's move.So lets try to understand what's going on in the following position :

1. White has just played 21 Rxd5 
2. Black's Queen is under attack.If Black does not moves his Queen White will happily capture the Queen winning the Queen for nothing.
3. White's rook on c1 is the only unsupported piece on the board which might be vulnerable to some tactics.Always Remember L.P.D.F which stands for Loose Piece Drops Off !!
4. White has pawns on h2,g2,f2 which might make his back rank vulnerable.
5. Black does not have the bishop on g7, which might make his king weak too.White also has a hidden threat of playing the rook on d6 (with idea of Qf6) or d7 (with idea of Rf7) when Black moves his Queen
6. White now has a threat of capturing the Queen..So in order to deal with the threat Black has to 

                  1.Move his Queen to some safe square might be on Qb6.

                  2.Capture the rook by Qxd5 

                  3.Obstruct the threat with b5

                  4.Try some counter attack by making more or equal                              dangerous threats like Nd4

So by studying the position we came to the conclusions that Black has following Candidate moves 

1. 21...Qb6
2. 21...Qxd5
3. 21...b5
4. 21...Nd4                                                                                                                                             

      Now lets start the Calcuation and process of elimination.

The second option of 21...Qxd5 looses material and there is no further way to take advantage or gain lost material back ...So we will eliminate second option of 21...Qxd5 from our candidate list..So now we are left with following Candidate move

1. 21...Qb6
2. 21...b5
3. 21...Nd4                                                                                                        

Let's start working with the moves                                                                                                                                                                 1. 21....Qb6

White will play his Rook on d6 threatening 23.Qf6 followed by 24.Rd7 which looks almost winning for White.

Please Note that playing Rook on d7 will gives Black an option of repelling White attack by exchanging his Rook with ...Rc7.

                                   Position after 21...Qb6 22.Rd6

2. 21...b5

Its a same story with 21...b5 White will play 22.Rd6 with the same threats of Qf6 followed by Rd7 giving him a better position.

                                 Position after 21...b5 22.Rd6

This leaves us to the last option and the most intresting variation of all these 21...Nd4..Please Calculate all the options for White before playing this move