2.e5 Nd5 3.d4 d6

The idea

After 2.e5 Nd5 3.d4 d6, Black has started undermining the pawn on e5. White chooses between 4.Nf3, the Modern Variation, and 4.c4, chasing the knight again and leading to the Exchange Variation or the Four Pawns Attack.

Plans for White

Choose between solid development with Nf3 and Be2, and grabbing space with c4. White's e5 pawn is the main asset and the main target, so support it with Nf3 or f4, or exchange it on d6 on White's terms.

Plans for Black

Keep hitting e5 with ...dxe5 or ...Nc6, and develop the light-squared bishop to g4 or f5 before ...e6. Black wants White's pawns to become targets, not to cramp Black's pieces.

Watch out for

For Black, f7. In the Balogh Variation, 4.Bc4 dxe5 5.dxe5 Nb6?? loses the queen to 6.Bxf7+, as the trap shows; Wikipedia gives this line.

For White, the e5 pawn. If Black takes on e5 and White can't recapture with a piece, Black's pressure on the center grows.