Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...a6/4. Ba4/4...Nf6

=Ruy Lopez Main Line= Black's move 4...Nf6 threatens White's e-pawn. White may defend it directly with but both moves interfere with White's natural plan of playing c3 and d4. Without playing c3 and d4 to create some central tension it will be difficult to generate open files for the rooks; additionally White's star attacker currently residing on a4 is liable to get swapped off if it does not have the option of retreating to c2.
 * 5.d3, the Anderssen Variation or
 * 5.Nc3 transposing to the Four Knights, Spanish Variation


 * 5.Qe2, the Wormald Attack, retains the option of c3 and d4, and introduces the medium-term plan of shifting the king's rook across to d1 in a single move after White castles kingside. The downside is that it places the queen on a file that's very likely to get opened, or at least half-opened. Put your rooks on open files and your queen on a closed one!

The main line, and a more subtle way of maintaining material equality, is 5. O-O, after which Black can grab a pawn but cannot hold on to it.

Theory table

 * 1.e4 e5 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6