Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. d3

Ruy Lopez, Schliemann Defence

Common moves:
4...fxe4 - only move

The move 4.d3 defended e4 but also threatened to snaffle the f5-pawn into the bargain, because now Black won't have the instant rejoinder ...e4.

4...fxe4 is the only move to justify the Black pawn thrust.

Wikipedia has related information at Ruy Lopez

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Chess Opening Theory

1. e4 e5
Open game

2. Nf3

With 2...Nc6:

With other 2nd moves:

2. Other

1. e4 c5
Sicilian defence

With 2...Nc6:

With 2...d6:

With 2...e6:

With other 2nd moves:

Anti-Sicilians

1. e4 ...other:

Other

1. d4 d5
Closed game

2. other

1. d4 Nf6
Indian defence

With 2...e6:

With 2...g6:

With other 2nd moves for Black:

2. other:

1. d4 f5
Dutch defence

1. d4 ...other:

Flank

Unorthodox