Jaenisch Variation

Petrov's Defense

1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nf3 Nxe4 5.d4 d5 6.Bd3 Nc6 7.O-O Be7 8.c4 Nb4 9.Be2

Black develops with Nc6 and Be7; after White's c4, the knight jumps to b4 to hit the bishop on d3.

Fun fact: The 8.c4 Nb4 line is credited to Carl Jaenisch and has been popular for over a century. Source

The idea

After 6...Nc6 7.O-O Be7 8.c4, White attacks d5. Black answers with 8...Nb4, hitting the bishop on d3, and after 9.Be2 the bishop is driven off its best diagonal. The same position can also come from 6...Be7 7.O-O Nc6 8.c4 Nb4.

Plans for White

After 9.Be2, the plans are Nc3 to challenge the knight on e4, a3 to drive away the knight on b4, and cxd5 to open the c-file. White plays for a small, lasting edge in a symmetrical-looking position.

Plans for Black

Castle, then develop with ...Bf5 or ...Be6, and keep the knight on e4 well supported. If White plays a3, the knight on b4 usually goes back to c6. Black's pieces are active, and the position is very solid.

Watch out for

For Black, the knight on b4 can get stranded. After a3, it must retreat, so make sure c6 is available.

For White, losing the d4 pawn. With Black's knights on b4 and e4 and pressure from ...Bf5, White's center can come under fire if White plays too slowly.