Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nf3/3...Nf6/4. Nc3/4...dxc4/5. a4/5...Bf5

Black develops their light-squared bishop. This is possible now because the threat of Qb3 is no longer present thanks to the pawn on c4.

White should try to regain their pawn, which can be done in two ways.

6. e3 is the Dutch variation. This is the classical main line. White tries to capture the pawn with 7. Bxc4 and continues the game by castling and preparing to play e4.

6. Ne5 is the central variation. Instead of recapturing the pawn with the bishop, White tries to do so with their knight instead. This is White's most ambitious option and leads to sharper positions.

6. Nh4, the Bled attack, is the chief sideline for White. Instead of going after the pawn, White first tries to get rid of Black's light-squared bishop. This move can be played in the Dutch variation too, and the positions may transpose into each other.

1. d4 d5 2. c4 c6 3. Nf3 Nf6 4. Nc3 dxc4 5. a4 Bf5

Wikipedia has related information at Slav Defense.

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