Chess Opening Theory/1. e4/1...c5/2. Nf3/2...d6/3. d4/3...cxd4

With 3...cxd4, Black breaks up White's two-pawn centre and achieves a majority of central pawns and a semi-open c-file, expecting the pawn to be recaptured on the next turn.

While White could play in a gambit-fashion with 4. c3, transposing to a delayed Smith Morra gambit, in the large majority of cases, White recaptures the pawn.

4. Nxd4 is the main move, centralizing the knight, which eyes key squares such as b5 and f5. This was one of the main purposes of 2. Nf3, allowing White to recapture this d4-pawn with the knight rather than the queen, thus saving valuable tempi.

4. Qxd4 is a minor sideline, known as the Chekhover Sicilian. This centralizes the queen, but is less common as Black can play 4...Nc6 to chase away the queen. In order to avoid losing tempi, White usually must give away the bishop pair with 5. Bb5, and after 5...Bd7 6. Bxc6 Bxc6 Black gains the advantage of the two bishops. Nevertheless, this line is playable and could be used as a surprise weapon if Black is only prepared for the lines after 4. Nxd4.

1. e4 c5 2. Nf3 d6 3. d4 cxd4

Wikipedia has related information at Sicilian Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.