Chess Opening Theory/1. d4/1...Nf6/2. c4/2...g6/3. Nc3/3...Bg7/4. e4/4...d6/5. Nf3/5...O-O/6. Be2/6…e5

With …e5, Black attacks the centre and opens up the e7 square for the b8 knight. White usually respond with 8. O-O, but there are alternatives. 8. d5 is the Petrosian variation where white tries to suffocate black. 8. dxe5 is the Exchange variation.