411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...e5/2. f4/2...exf4/3. Nf3/3...d6/4. d4/4...g5
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The most popular move in this reason. It serves 2 purposes. 1 is to defend the f4 pawn temporarily, and the next purpose is to march to g4 next move and attack the knight. White can continue in multiple ways here, such as Nc3 and h3