Main Line (4.Nxd4)

The idea

After 4.Nxd4, White has a knight in the center and a pawn on e4. Black's main replies are 4...Bc5 (the Classical Variation), 4...Nf6 (the Schmidt Variation, which leads to the Mieses Variation) and 4...Qh4 (the Steinitz Variation).

Plans for White

Support the knight on d4 with Be3 and c3, or trade it on c6 when that damages Black's pawns. Develop quickly and castle; White's space makes the position easier to play.

Plans for Black

Attack the knight on d4 and the e4 pawn at once. Black's development is easy, so the goal is active pieces and the freeing ...d5 when possible. The knight on c6 and the queen are Black's best attackers of d4.

Watch out for

For Black, trading on d4 too readily. After 4...Nxd4 5.Qxd4, White's queen gets a strong, central square and Black has spent time without developing.

For White, the knight on d4 can become a target for ...Bc5, ...Qf6 and ...Nf6, so make sure it stays protected.