411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Bxc6/4...dxc6/5. d4/5...exd4
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Ruy Lopez: Exchange Variation rd bd qd kd bd nd rd pd pd pd pd pd pd pd pd pl nl pl pl pl pl pl pl rl nl bl ql kl rl Ruy Lopez
Ruy Lopez: Exchange Variation
The best move for white here is 6.Qxd4. This leads to 6.- Qxd4 7.Nxd4. This line is thought to slightly favor black.
6.Nxd4?! is bad because of 6.- c5! and after 7.Nb5 or 7.Nb3, black shall play Qxd1+!, which will give him an advantage.