411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...c5/2. c3/2...Nf6/3. e5/3...Nd5/4. d4
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Sicilian, Alapin rd nd bd qd kd bd rd pd pd pd pd pd pd pd pd nd pl pl pl pl pl pl pl pl rl nl bl ql kl bl nl rl
4.d4
This move gains central space and connects with the e-pawn. White has created a pawn chain in the center and is now threatening c4 followed by d5, when Black would really suffocate by his lack of central space. This pawn controls White's center.