Chess Opening Theory/1. e4/1...e5/2. f4/2...exf4/3. Nf3/3...g5/4. Bc4/4...g4

= King's Gambit Accepted, 4...g4= With this move, Black reveals the full purpose of 3...g5 — not only did the g pawn protect Black's extra f pawn, it also threatened to dislodge White's sole defender of the h4 square, the knight on f3. White must now reconcile the dual threats of 5...gxf3 and 5...Qh4+.

White can safely ignore the threat with 5.O-O!, sacrificing his knight in order to move his king to safety, develop his rook to the semi-open f file, and rapidly develop his queen to f3. If White so chooses, he can sacrifice a second piece to move the Black king to the f-file, giving White excellent practical chances.

Because the Muzio and double Muzio gambits give Black an overwhelming number of problems to solve over the board, most players avoid this line completely and simply play 4...Bg7 with a good game.

White's alternative fifth moves, 5.Nc3!? and 5.Bxf7+, are relics of a bygone era and thus rarely seen in tournament play, also Ne5 allows Qh4+.

Theory table
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1.e4 e5 2.f4 exf4 3.Nf3 g5 4.Bc4 g4

/5. Ne5/