Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nf3/3...Nf6/4. Nc3/4...a6

= a6 Slav = The move 4...a6 introduces the a6 (or Chebanenko or Chameleon) Slav. Though it seems a small move, it prepares ideas of ...b5 (either to attack White's c4 pawn or to shore up Black's after ...dxc4) and eases the problems of developing Black's light-squared bishop, as now Qb3 can be met with ...Ra7. The weakening of the b6 square, however, invites White to simply push - 5. c5 is the most common response, but White can also play 5. e3, solidifying his d pawn and adding his light-squared bishop to the fight, while accepting 5...b5. Less common are 5. a4, preventing ...b5, and 5. cxd5, releasing the central tension but leaving Black to show what purpose ...a6 now serves.

Theory table
1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 a6