Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nf3/3...Nf6/4. Nc3/4...a6/5. c5

= a6 Slav = White plays 5. c5 (the Advance System) to control the weak b6 square and forestall any ...dxc4 or ...b5 ideas. If he can make this space advantage permanent, he will have a very comfortable game, so Black needs to start planning a pawn break on either b6 or e5. To support this, the main move is 5...Nbd7, developing the queen's knight to its best available square and preparing whichever break Black chooses. The main alternatives are 5...Bf5, developing the bishop before it gets blocked in by ...Nbd7, and 5...g6, preparing a fianchetto. 5...Bg4 is also played occasionally, with the idea of pressuring the e5 square indirectly, but there is no reason White cannot simply occupy it at once with 6. Ne5.

Theory table
'''1.d4 d5 2.c4 c6 3.Nf3 Nf6 4.Nc3 a6 5. c5'''