Chess Opening Theory/1. d4/1...d5/2. c4/2...e6/3. Nc3/3...Nf6/4. cxd5/4...exd5

4...exd5

With this move, Black frees up his light squared bishop, which was originally blocked with the move 2...e6. In addition, the classic Carlsbad pawn structure is entered, in which White has a pawn majority in the centre, and Black has a pawn majority on the queenside.

White wants flexibility on where to place the knight on g1 in this position. In the cases in which White goes for the minority attack, a knight on f3 is usually preferred, and in cases in which he prefers a central expansion with f3 and e4, a knight on e2 is preferred. As a result, White usually prefers to develop the light squared bishop first (almost always to d3), so ...e3 is played before deciding on a placement for the knight. However, playing 5. e3 immediately would block in the dark squared bishop, so White almost always continues with 5. Bg5, pinning the knight on f6 and adding pressure to the pawn on d5, whilst also getting the bishop outside the pawn chain. With this move, White threatens to double Black's pawns with 6. Bxf6, as 6...Qxf6 would lose a pawn to 7. Nxd5.