2...Nf6 (Modern Variation)

The idea

After 2.exd5 Nf6, Black doesn't take back at once. The knight will recapture on d5 later, so Black's queen never becomes a target. White decides between 3.d4, the most common choice, and trying to hold the pawn with 3.c4.

Plans for White

With 3.d4, White builds the center and lets Black recapture, aiming for a space advantage. With 3.c4, White tries to keep the pawn; Black's gambits then give quick development in return, so White must catch up safely.

Plans for Black

Recapture with ...Nxd5 if White allows it, and develop with ...Bg4 or ...Bf5, ...e6 and ...Nc6. Against 3.c4, Black gambits with ...c6 or ...e6 to open lines for the pieces. Black's knight can be hit by c4, so it needs a good square.

Watch out for

For White, holding on too hard. After 3.c4, extra pawn moves fall behind in development, and Black's pieces become active fast.

For Black, the knight on d5. After ...Nxd5, c4 hits it, and if it goes to the wrong square it can end up offside. In the Kiel Variation, 4.c4 Nb4, Wikipedia notes 5.a3 or 5.Qa4+ and 6.a3 gives White a large advantage.