Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. Nd2/3...c5/4. exd5

White takes the d5 pawn with 4. exd5, aiming to resolve the tension occurring in the center.

Black has two ways to capture back the pawn:

4...exd5 is the old main line, and what used to be played in Karpov vs Korchnoi World Chess Championship matches in the late 1970s. Black frees up their light squared bishop, but will also probably have to accept an isolated queen's pawn (IQP).

4...Qxd5 is the Chistyakov Defence, and has now become the modern main line. Black takes advantage of the fact that White can't play Nc3 to chase away the queen from d5. Black will eventually win the d4 pawn, but White aims to win it back whilst gaining a slight lead in development.