Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...Nf6/4. O-O/4...Nxe4/5. d4/5...a6

= Ruy Lopez:Open Game =

White's threatened bishop has five options, but any retreat along the f1-a6 diagonal it travelled on move 3 constitutes a loss of tempo, while Bxc6 gives a bad Exchange Variation where the lack of a fixed centre makes Black's bishop pair a fearsome force. So White is more or less forced to transpose into the Open Variation with Ba4.

This is a good example of a move-order trick. Fans of the Open Variation can avoid a whole world of nonsense (Exchange Variation, Worrall Attack, 5. d3) by playing 3...Nf6 and transposing later.