Chess Opening Theory/1. e4/1...e5/2. d4/2...exd4/3. c3/3...dxc3/4. Bc4/4...cxb2/5. Bxb2/5...Nf6/6. Nf3

= Danish Gambit Accepted, Classical Defence =

6. Nf3
6…Nxe4? would be a great move to teach chess students about the value of development  over pawn-grabbing. The simple 7. O-O sets in motion an overwhelming attack.

Part of the reason White wants to play 7. O-O is that it deprives Black of the bishop check on b4, which is otherwise a free developing move. So it makes sense that Black's most popular move is the immediate  6...Bb4+ followed by castling, and that a less popular but  higher-scoring alternative is 6...d5 to distract White from  castling by threatening a piece. After 6...d5 7.exd5 Black has the nice d6 square for the bishop, Open Ruy Lopez style.

Black can opt for passive defence with 6...Be7 or 6...d6, or for a transposition to the Göring Gambit with  6...Nc6.