This is a good square for the dark-squared bishop.

Black aims to penetrate the White position along either the c8-h3 or the a6-f1 diagonal; this move prepares that without tipping Black's hand which one it will be.

Preparing to castle queen-side. Castling king-side is out of the question, on account of the threat of 20. . . . Nf6; 21. Kh1, Qh3 (threatening mate on g2); 22. Rg1 (forced), Nf6-g4 (threatening mate on h2) etc.

With White's king still centred, Black has sufficient initiative and development to mount a direct attack on the White pawn centre.

If 21. d4xBc5 then Nxc5 and White still cannot castle, for if 0-0-0 then . . . Nb3+ wins the queen. 0-0 leads to a quick mate, explained above. On any other 22d move by White, . . . Nd3 and White is paralyzed.

It is time to challenge White's best-placed piece.

This would be a fine square for the knight, if only it could stay there.

With White's active pieces off the board and his king still centred, the position is ripe for cracking open to activate files for Black's rooks.

White's pieces are committed to holding f1; this allows the Black queen to penetrate the position at will on the king-side or the queen-side on either of the two diagonals mentioned earlier, c8-h3 or a6-f1.

This blocks the Black queen's entry on the king-side.