411 REFERENCE · Wikibooks
Chess Opening Theory/1. d4/1...d5/2. Nc3/2...Nf6/3. Nf3/3...Nc6
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3...Nc6 — Queen's Four Knights System
After 1. d4 d5 2. Nc3 Nf6 3. Nf3, Black plays 3...Nc6, developing the remaining knight.
4. Bf4, develops the bishop actively.
4. e3, builds a solid center.
4. Bg5, hits the knight on f6 and increases pressure.
Downsides: The main drawback is that both sides have committed their knights early, which can make the position somewhat cramped.
The main line is 1. d4 d5 2. Nc3 Nf6 3. Nf3 Nc6 4. Bf4 e6.
The Queen's Four Knights System is characterized by all four knights being developed early: Nc3, Nf6, Nf3, and Nc6.
Also see: 4 Knights Game