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

= Slav Accepted Alapin Variation = The advance of the a-pawn prevents Black from controlling b5 which in turn leaves the c4 pawn available for capture.

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