Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nf3/3...Nf6/4. Nc3/4...dxc4/5. a4

The advance of the a-pawn prevents Black from playing 5...b5, which in turn leaves the c4 pawn available for capture. This is White's most popular and ambitious response. Over the last century, there have been four major lines played by Black.

5...Bf5, the Czech variation, is by far the most testing and theoretical line. The bishop eyes at the center and makes it harder for White to play e4.

5...Bg4 is the Steiner variation. It is played quite often at the amateur level, but requires Black to understand the positions well to be played successfully. White's knight isn't pinned and can move to e5 with a tempo on the bishop.

5...Na6 is the Smyslov variation. While it looks odd at first sight, the goal of the move is quite understandable: Black wants to maneuver the knight to b4, where it can't be chased away by pawns.

5...e6 is the Soultanbeieff variation. Black opts to play in more of a Semi-Slav style and temporarily locks the light-squared bishop behind the pawn chain.

1. d4 d5 2. c4 c6 3. Nf3 Nf6 4. Nc3 dxc4 5. a4

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