[FEN ""] 1. e4 e5 2. Nf3 Nc6 3. Bb5 a6 4. Ba4 Nf6 5. O-O Nxe4
Why is 6. Re1 not played, prefer players 6. d4 only? Please answer with explanation and variations so that it could help all players.
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6.Re1 is considered inaccurate because black has the strong reply 6...Nc5!. Nc5 forces white to part with the bishop pair, and although white will regain the pawn, white cannot claim any advantage. Black will have a slight lead in development after, for example,
7.Bxc6 dxc6 8.Nxe5 Be7 9.d4 Ne6 because white will have to consolidate the d4 pawn at some moment.
6.d4 has several advantages. Black's king is still in the center and white has a lead in development, so it is a principled response to try to open the center. d4 keeps white pieces off of the c5 square (at least until white plays dxe5). Finally, in many lines of the Open Ruy, white is quite happy to keep the rook on f1 where it defends the f2 square. For example, consider the following:
[Fen ""] 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Nxe4 6.d4 b5 7.Bb3 d5 8.dxe5 Be6 9.Nbd2 Bc5
This is one of the main starting positions of the Open Ruy, white is immediately subject to pressure on f2, and the rook on f1 gives white a moment to complete development with c3 and Bc2 (for example).
After 6.Re1, black equalizes comfortably after 6....Nc5, as he wins the bishop pair. For example, 7.Bxc6 dxc6 8.Nxe5 Be7 9.d4 Ne6 10.c3 0-0.
Instead, 6.d4! is the main line, after which 6....b5 7.Bb3 d5 8.dxe5 Be6 is almost forced for black. The resulting position is the tabiya of the Open Spanish.
[StartPly "10"] [FEN ""] 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Nxe4 6.Re1 (6.d4! b5 7.Bb3 d5 8.dxe5 Be6) Nc5 7.Bxc6 (7.Nxe5 Be7) dxc6 8.Nxe5 (8.d4 Ne6) Be7 9.d4 Ne6 10.c3 O-O