2...Bb7 3.Nc3 e6

The idea

After 2...Bb7 3.Nc3 e6, White hasn't committed to e4 yet. White can play 4.e4 now, inviting the Hartlaub Gambit, or 4.Nf3, after which Black's ...Bb4 leads toward a Nimzo-Indian kind of game.

Plans for White

Decide how to build the center. 4.e4 is the most ambitious, taking the whole center at the cost of allowing ...f5. 4.Nf3 is quieter, aiming for a sound position with Qc2 and a3 against ...Bb4.

Plans for Black

Pin the knight with ...Bb4 and hit e4 with ...f5 or ...Nf6. Black's bishop on b7 and the pin on c3 both work against e4, and Black wants to stop White from playing it on good terms.

Watch out for

For Black, the pin can be broken. Qc2 and a3 put the question to the bishop on b4, and if Black isn't ready to take on c3, the bishop can be pushed back.

For White, doubled pawns. If Black takes on c3 and White recaptures bxc3, the c-pawns can be targets, so White must get play in the center for them.