411 REFERENCE · Wikibooks
Chess Opening Theory/1. Nf3/1...d5/2. g3/2...c5/3. Bg2/3...Nc6/4. d4
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Reversed Grünfeld
With 4. d4, White enters the reversed Grünfeld where play can be very sharp. The d-pawn immediately confronts Black's centre and forces a response.
Black may respond with /4...cxd4/, entering a reversed exchange Grünfeld. However, they may also respond with /4...e6/, discovering the f8-bishop's defense of the c4-pawn. The most common response is /4...Nf6/.