Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. d3/4...fxe4/5. dxe4/5...Nf6/6. O-O/6...d6/7. Nc3

Having put the pawn on d6, the only square available for the f8-bishop is e7, so it might as well go there immediately with 7...Be7. The c8-bishop still has options, and the option of breaking the pin on the c6-knight with Bd7 may be particularly useful, so it typically stays put for now.

7...Bg4 is also played, inviting White to transpose to 7.Qd3 lines with 8.Qd3 which gets out of the pin.