411 REFERENCE · Wikibooks
Chess Opening Theory/1. f4/1...e5/2. fxe5/2...Nc6/3. Nf3/3...g5/4. d4
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Bird's Opening rd bd qd kd bd nd rd pd pd pd pd pd pd nd pl pd pl nl pl pl pl pl pl pl rl nl bl ql kl bl rl
4.d4
This is a common move in the From's Gambit for White, defending e5 and attacking g5 at the same time. 4.h3 was weakening, but also playable. However, it was not in the spirit of the From's Gambit.
Black can now continue his plan with 4...g4.Lakdawala, Cyrus. Bird's Opening: Move by Move. Everyman Chess, 2015.
Theory table
+45d4
g4Ng5
d6 +/=
References