Chess Opening Theory/1. e4/1...Nf6/2. e5/2...Nd5/3. d4/3...d6

= Alekhine's Defence = At that point, it's unwise for White to take the d6 pawn, which means giving up the space advantage and allowing Black to build his own pawn center.

While /4. c4/ is tempting, the modern variation /4. Nf3/ gets better results, making harder for Black to equalize.

Theory table
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1.e4 Nf6 2.e5 Nd5 3.d4 d6