Two Knights Defense

The Two Knights Defense, historically also called the Prussian Defense, answers 3. Bc4 by developing the king's knight and attacking the e4-pawn at once. The price is that the knight on f6 cuts the queen off from g5, so White can bring a second piece against f7 before Black has castled. That is the main difference from 3...Bc5, which keeps g5 covered, and it makes the Two Knights the more combative of Black's two main replies. Bronstein considered the word "defense" misleading and proposed the name Chigorin Counterattack.

The Knight Attack, 4. Ng5, is the traditional test. Tarrasch dismissed it as a beginner's move because it moves a piece twice before development is complete, but it has been used by Steinitz, Fischer, Karpov, Kasparov and Anand, and Morozevich and Short revived it in the 1990s. Black's standard answer is 4...d5 5. exd5 Na5, the Polerio Defense: after 6. Bb5+ c6 7. dxc6 bxc6, a line Chigorin championed, Black is a pawn down but ahead in development and has the initiative. Other fifth moves are 5...Nd4 (Fritz Variation) and 5...b5 (Ulvestad Variation), which often transpose into one another. The natural recapture 5...Nxd5 is considered too dangerous because of 6. Nxf7, the Fried Liver Attack, and 6. d4, the Lolli Attack. Black can also ignore the threat with 4...Bc5, the Traxler Counterattack, which strikes at f2 and produces wild complications; 4...Nxe4, the Ponziani-Steinitz Gambit, is held to be unsound because of 5. Bxf7+ Ke7 6. d4.

4. d3, the Modern Bishop's Opening, is the most frequent move. It protects e4, frees the c1-bishop and steers toward a maneuvering game; after 4...Bc5 it becomes the Giuoco Pianissimo, and with c3 and a bishop retreat to c2 via b3 the positions resemble the Ruy Lopez. 4. Nc3, transposing to the Four Knights Game, runs into the fork trick 4...Nxe4 5. Nxe4 d5. The Open Variation, 4. d4, gives up the e-pawn for central play and normally continues 4...exd4, a Scotch Gambit position; 5. Ng5 is the Perreux Variation, while 5. O-O Bc5 6. e5 is the Max Lange Attack. 4. c3 leads to the Neumann Gambit.

Polerio recorded the opening around 1580, and its theory grew extensively in the 19th century. Chigorin, Keres, Tal and Spassky are among its notable exponents. In online play 4. d3 and 4. Ng5 are nearly equally popular, results are slightly in White's favor with very few draws, and the engine assesses the starting position as roughly balanced.