Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. d4/4...exd4

= Two Knights Defence - Classical Variation =

4...exd4
Black plays exd4. Black is a pawn up but has a number of weaknesses the square e5, the Knight on f6 that can be pinned and also the weak f7 square. This all adds up to giving White a number of options. They are.
 * 5. O-O the classical variation.
 * 5. e5 the modern variation.
 * 5. Ng5 a misguided direct attack on f7.
 * 5. Nxd4 rather normal move.
 * 5. Bg5 pinning the Knight, but the pin can soon be undone, leaving no advantage for White.
 * 5. Qe2 this supports e5, although e5 can be played immediately anyway.
 * 5. c3 fails to 5...Nxe4 where Black has the nice d5 available and time to block any pins on the e-file against the King.

Theory table
'''1. e4 e5 2. Nf3 Nc6 3. Bc4 Nf6 4.d4 exd4'''