411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...c6/2. Nc3/2...d5/3. Nf3/3...Nf6
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Caro-Kann Defence: Two Knights Attack B10
3...Nf6 · Caro-Kann Defence: Two Knights Attack
3...Nf6 adds pressure to the d pawn, and has the advantage for Black of being played far less often than the main line 3...Bg4, or 3...dxe4. White only has one good response, 4. e5.
Attempting to defend the pawn with 4. d3? is inaccurate, as after Black captures and the resulting exchanges, White either loses a pawn, or loses the right to castle.
Exchanging with 4. exd5 is playable, but is effectively an inferior version of the Exchange Variation for White.
Theory table
4. e5 4. Qe2 4. exd5
Caro-Kann