Chess Opening Theory/1. e4/1...c5/2. Nf3/2...Nc6/3. d4/3...cxd4/4. Nxd4/4...Nf6/5. Nc3/5...d6

=Classical Sicilian=

5...d6
Black's move 5...d6 protects the recently developed knight from being attacked. A pawn move is thus in order, since both knights are developed to good squares, and the Bishop on c8 now has the option, but not the obligation, to develop to a good square once white makes their intentions known.

Of all the responses for white, Bg5 is perhaps the best, and should be expected from a strong opponent. It pins the king knight, prepares for queenside castling, and it is the prelude to the Richter-Rauzer Attack (named after Vsevolod Alfredovich Rauser ).

Theory table
1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 d6