The aptly named Two Knights Defense is reached if Black plays Nf6 on his third move. Compared to other Black responses, Nf6 does the most to fight for the initiative, as it immediately attacks White's e4 pawn.
White has several acceptable responses, including d3, d4, and the complex Ng5, which is often rejected as a beginner's move. Several fun and exciting lines -- though rarely played by strong players -- can begin after the latter move, such as the Fried Liver Attack and the Traxler Variation.
