Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...f5/4. d4

This variation counterattacks in the center by challenging the e-pawn, like the Vienna Gambit with 1. e4 e5 2. Nc3 Nf6 3. f4 d5.

4. ... exd4
If black takes the d-pawn, white's most common move according to Lichess's Master games explorer is e5, pushing the pawn forward and taking control of the f6 and d6 squares.

4. ...fxe4
If black takes the e-pawn, white's most common move according to Lichess's Master games explorer is Nxe5, pushing the knight into black territory, where it controls the c6, g6, f7, and d7 squares. From here the most common moves are 5. ... Nxe5 6. dxe5 c6.

4. ... Nxd4
If black takes the d-pawn with the knight (which resembles the Ruy Lopez Bird Variation), it hits the bishop on b5, so Nxe5 is not an option. 5. Nxd4 is the most common move, followed by 5. ... exd4 O-O 6. c6.

4. ... Nf6
If black plays Nf6, dxe5 is the most common move, followed by 5. ... Nxe4.