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

Black now kicks White's bishop. Removing its pressure from Black's knight will allow Black to play ...d5, expanding in the centre and defending the knight on e4, without dropping a pawn due to the pin.

Any retreat along the f1-a6 diagonal the bishop travelled on move 3 constitutes a loss of tempo and allows ...d5. The two serious possibilities are 6. Bxc6 or 6. Ba4.

6. Ba4 is a transposition to the Open Spanish. Reaching the Open Spanish via the Berlin defence is a move order trick that avoids sidelines like the Wormald attack.

But White doesn't have to transpose into an Open Spanish if they do not want to: 6. Bxc6 dxc6 enters something like the Exchange variation, though there really is a threat of Nxe5 this time. Rather than recover the pawn straight away, White's better line is 7. Qe2, pressuring the e-file while unpinning the d-pawn to prevent a queen trade.

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

Wikipedia has related information at Ruy Lopez.