Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...Nf6/4. d3

4. d3 defends White's e4 pawn. This avoids the mainline of the Berlin defence, 4. O-O, where White temporarily sacrifices the e4 pawn.

Now that White's e4 pawn is protected, this increases the threat to Black's e5 pawn. If for example 4...a6?, White can trade off the defending knight and win the pawn (5. Bxc6 dxc6 6. Nxe5), and Black's usual 6...Qd4? tactic no longer forks anything.

Black can choose to defend the pawn directly or tactically:

4...d6, to defend the e5 pawn directly, is playable but relegates the kingside bishop to a supporting role on e7.

4...Bc5 is the mainline. This pressures White's f2 pawn and sets up a new tactical riposte: now if White trades off the knight and takes the pawn (5. Bxc6 dxc6 6. Nxe5), then 6...Qd4! forks the knight with the threat of Qxf2#. White's usual response is 5. c3, controlling the d4 square and so preventing Qd4.

4...Ne7, the Mortimer variation, is a sideline. This gambits the e5 pawn, but if White takes it, they fall into the devious Mortimer trap: 5. Nxe5 c6 6. Bc4 Qa5+!, forking the knight with check.

For explanation of theory tables, see theory table and for notation, see algebraic notation.

1. e4 e5 2. Nf3 Nc6 3. Bb5 Nf6 4. d3

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