The Two Knights Defense is a chess opening that begins with the moves:

First recorded by Polerio (c. 1550 – c. 1610) in the late 16th century, this line of the Italian Game was extensively developed in the 19th century. Black's third move is a more aggressive defense than the Giuoco Piano which would result from 3...Bc5. In fact, David Bronstein suggested that the term "defense" does not fit, and that the name "Chigorin Counterattack" would be more appropriate. The Two Knights has been adopted as Black by many aggressive players including Mikhail Chigorin and Paul Keres, and World Champions Mikhail Tal and Boris Spassky. In modern grandmaster play, 3.Bc4 is far less common than 3.Bb5 and the more solid 3...Bc5 is the usual reply, so the Two Knights Defense is infrequently seen. It remains popular with amateur players. The theory of this opening has been explored extensively in correspondence chess by players such as Hans Berliner and Yakov Estrin.