411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...d6/2. d4/2...Nf6/3. Nc3/3...g6/4. Bg5/4...Bg7
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Pirc Defence
4...Bg7 · Pirc Defence
4. Bg7 is the most common and a natural response, both protecting Black's pawn structure and taking the natural square for the bishop. White has numerous responses here, 5. Qd2 being the most common, as well as 5. f4. 5. e5 is a sound line that also invites Black into a possible trap.
Theory table
5. Qd2 5. f4 5. e5 5. Qe2 5. Nf3
References