Classical Variation: 5...Nc6
This variation can arise from two different move orders: 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 Nc6, or 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 d6. Black simply brings his knight to its most natural square, and defers, for the moment, the development of his king's bishop.
White's most common reply is 6.Bg5, the Richter-Rauzer Attack. The move 6.Bg5 was Kurt Richter's invention, threatening to double Black's pawns after Bxf6 and forestalling the Dragon by rendering 6....g6 unplayable. After 6...e6, Vsevolod Rauzer introduced the modern plan of Qd2 and 0-0-0 in the 1930s. White's pressure on the d6-pawn often compels Black to respond to Bxf6 with ...gxf6, rather than recapturing with a piece (e.g. the queen on d8) that also has to defend the d-pawn. This weakens his kingside pawn structure, in return for which Black gains the two bishops, plus a central pawn majority, though these assets are difficult to exploit.
Another popular variation is 6.Bc4, which brings the bishop to an aggressive square. Black usually plays 6...e6 to limit the range of White's bishop, but White can eventually put pressure on the e6-pawn by pushing his f-pawn to f5. After the moves 7.Be3 Be7, White can either castle kingside (the Sozin Attack, named after Russian master Veniamin Sozin, who originated it in the 1930s), or queenside with 8.Qe2 and 9.0-0-0 (the Velimirović Attack). Instead of 6...e6, Black can also try Benko's move 6...Qb6, which forces White to take a decision over the knight at d4, and typically leads the game into more positional lines than the razor-sharp, highly theoretical Sozin and Velimirovic variations. 6.Be2 allows Black to choose among 6...e5, the dynamic Boleslavsky Variation; 6...e6, transposing to the Scheveningen Variation; and 6...g6, transposing to the Classical Variation of the Dragon.