411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...Nf6/5. O-O/5...Nxe4/6. d4/6...exd4/7. Re1/7...d5/8. Nxd4/8...Bd6/9. Nxc6/9...Bxh2/10. Kh1/10...Qh4/11. Rxe4
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Ruy Lopez:Riga Variation rd bd kd rd pd pd pd pd pd pd nl pd bl rl qd pl pl pl pl pl bd rl nl bl ql kl
Ruy Lopez, Riga Variation, Main Line
White takes the knight on e4, sacrificing the exchange to remove black's pressure on the f2 square. Now black has two options to retake: with 11...Qxe4 or 11...dxe4.
It turns out that 11...Qxe4, while possibly appealing because it attacks the a4 bishop, is an error. Moving the queen away lessens the pressure on the f2 square and leaves the h2 bishop hanging. White can counter the attack on his bishop with either 12. Nd4 (blocking the attack), or 12. Nc3 (defending the bishop).
Therefore, 11...dxe4 is the main line.
References
Ruy Lopez