Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. Nc3/3...Nf6/4. Bg5/4...Be7

=Classical variation= Because Nf6 is no longer pinned, Black threatens to win the e4-pawn. The attempt to defend the e4-pawn, e.g. by 5.f3?! or 5.Bd3?!, will still lose the pawn, because White's Bg5 is not defended, so Black can simply play 5...Nxe4! and win a pawn (6. Bxe7 Nxc3 7. Bxd8 Nxd1 8. Bxc7 Nxb2 -/+). That's why White either plays the e4-pawn to a safe square (Main Line - 5.e5), or eliminates an attacker of e4 (5.Bxf6). A safe alternative is a transposition to the Exchange Variation (5.exd5).

Theory table
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