2...e6 3.e4 Bb7

The idea

After 2...e6 3.e4 Bb7, White has pawns on c4, d4 and e4, and Black's bishop is already attacking e4. White chooses how to protect it: 4.Bd3, the most natural, 4.f3, or 4.Qc2. Daniel King notes that 4.Bd3 f5 was long the main line, though he calls ...f5 questionable today.

Plans for White

Protect e4 with a move that also develops, usually Bd3 or Nc3, and castle quickly. White's center gives more space, and if Black's pieces can't hold it under pressure, White can play d5 or e5 to gain more.

Plans for Black

Keep hitting e4. ...Bb4+ forces White to block, and ...f5 attacks e4 directly. With ...Nc6 and ...Nb4, Black can also go after the bishop on d3. Black's plans revolve around making White's center a target.

Watch out for

For White, letting e4 fall. If the pawn goes with tempo, the bishop on b7 hits g2 and White's position can collapse.

For Black, ignoring development. The pawn moves ...b6, ...e6 and ...f5 all cost time, so the pieces must follow quickly, or White's central break comes first.