411 REFERENCE · Wikibooks
Chess Opening Theory/1. d4/1...d5/2. c4/2...e6/3. Nc3/3...c6/4. Nf3
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Triangle system D31 Triangle system
4. Nf3
White develops the knight and leaves the opening choice to Black. The move /4...Nf6/ transposes into the Semi-Slav defence, while /4...dxc4/ enters the sharp Noteboom variation.
Theory table
Semi-Slav defence 4... Nf6 Noteboom variation 4... dxc4 unclear
References
Semi-Slav Defence The_triangle_setup