199 ( +1 | -1 ) Joanne has given one of the main lines...... of the Two Knights' Defence, but maybe a bit more detail is called for. After 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 ... White can choose between 4.Ng5; 4.d4 or even the more circumspect 4.d3. Some people prefer the non-committal 4.0-0.
There has long been a theoretical controversy whether 2.Ng5 (a "beginner's move according to Siegbert Tarrasch) or 4.d4 is the stronger. Personally, I go for the knight move.
After 4.Ng5, it would seem that Black has to do something about the attack against f7. The most usual response is 4...d5 5.exd5. Now, you might be wondering why Black doesn't just retake on d5 (5...Nxd5). The answer is that White gets a tremendous attack beginning with a knight sacrifice on f7: 4.Ng5 d5 5.exd5 Nxd5? 6.Nxf7+!? ... (A more circumspect approach is 6.d4) 6...Kxf7 7.Qf3+ Ke3 - Just about essential, to keep the d5-knight. Now White just piles up the pressure on the d5-knight:
8.Nc3 Nb4 9.a3 Nxc2+ 10.Kd1 Nxa1 11.Nxd5 Kd6 12.d4 ... and White has a powerful attack against the exposed Black king. This line is known as the Fegatello Attack, or the "Fried Liver" Attack.
Occasionally Black will just ignore the threat to his f7; 1.e4 e5 2.Nf3 Nc6 3.Bc3 Nf6 4.Ng5 Bc5!? - the Wilkes-Barre or Traxler Variation. White can choose which capture on f7, or just play quietly, 5.d4. Although the knight capture looks the more immediately profitable, the bishop take is thought probably to afford more long-lasting benefits. The problem with the knight capture is that it provokes a very violent response from Black:
5.Nxf7!? Bxf2+! 6.Kxf2 (Kf1 might be better) 6...Nxe4+ 7.Kg1 ... Just about essential. If, say, 7.Ke1, Qh5+ 8.g3 Nxg3 is terrible for White. 7...Qh4
Black has a strong attack, but White has good chances of survival.
Back to the main lines after 4.Ng5, Black has a couple of moves other than 5...Na5 to respond to the pawn capture on d5:
4.Ng5 d5 5.exd5 Nd4!? - the Fritz Variation 5... b5!? - the Ulvestad Variation Both are quite playable and interesting for Black. Maybe we can discuss those lines another time.
Suffice to say, The Two Knight's Defence is one of my favorite openings for Black. It is a pity I rarely get a chance to play it!