Key ideas
- After 4...dxc6, 5.O-O is the main move. It sets up a real threat to e5, because Nxe5 would no longer allow a strong ...Qd4. 5...f6 is the most solid defence; 5...Bg4, 5...Qd6 and 5...Bd6 are the other main tries.
- The structure explains the variation. White has four kingside pawns against three, while Black's four queenside pawns, with the doubled c-pawn, cannot create a passed pawn on their own. A pure king-and-pawn ending therefore favours White.
- White's typical plan is d4 to exchange the d-pawn for Black's e-pawn, then a quiet build-up. Black uses the bishop pair and free development, often with ...Bg4 or ...Be6, queenside castling and ...c5 to control d4.
- 4...bxc6 is a sound alternative that keeps Black's central pawns intact, at the cost of a slightly looser queenside.
Plans for White
Trade pieces, especially queens, and head for the ending; exchange the d-pawn for Black's e-pawn so the kingside majority is free to advance. Keep the knights active to blunt the bishop pair.
Plans for Black
Keep pieces on and use the two bishops and your quicker development. Queenside castling and kingside pawn advances such as ...g5 or ...h5 are common. Avoid simplifications that leave a pure pawn ending.
Watch out
Before White castles, 5.Nxe5? achieves nothing: 5...Qd4 forks the knight and the e4 pawn, and after 6.Nf3 Qxe4+ Black has regained the pawn with a comfortable game.
For the wider picture, see the Ruy Lopez guide.