Tarrasch Variation, Open System, Advance Line

Instead of 5...Nc6, which usually leaves Black with an isolated queen's pawn, Black advances 5...c4. This gains queenside space, but it gives up pressure on d4 and leaves d5 without pawn support. The most common reply is 6. c3. The engine's first choice, 6. b3, challenges the advanced pawn at once and scores better for White in practice. Engines give White a slight edge, and the line is seldom played.