Two Knights Defense
The Two Knights Defense is a chess opening that begins with the moves: First recorded by Giulio Cesare 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 (). Black invites White to attack his f7-pawn with 4.Ng5. If White accepts the offer, the game quickly takes on a tactical character: Black is practically to give up a pawn for the initiative. The complications are such that 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. Main variations . 4.Ng5. German master Siegbert Tarrasch called 4.Ng5 a "real duffer's move" ("ein richtiger Stümperzug") and Soviet opening theorist Vasily Panov called it "primitive", but this attack on f7 practically wins a pawn by force. Despite Tarrasch's criticism, 4.Ng5 has remained a popular choice for White at all levels. Main line: 4...d5. After 4...d5 White has little option but to play 5.exd5, since both the bishop and e4-pawn are attacked. Then Black usually plays 5...Na5 but there are other options: After 5...Na5, Paul Morphy would play to hold the gambit pawn with 6.d3. The  Morphy Variation (or Kieseritzky Attack) has not been popular, since it has long been known that Black obtains good chances for the pawn with 6...h6 7.Nf3 e4 8.Qe2 Nxc4 9.dxc4 Bc5. (Bronstein once tried the piece sacrifice 8.dxe4!? with success, but its soundness is doubtful.) Instead, White usually plays 6.Bb5+, when play usually continues 6...c6 (6...Bd7 is also possible) 7.dxc6 bxc6 8.Be2 h6. (The move 8.Qf3, popular in the nineteenth century and revived by Efim Bogoljubov in the twentieth, can be played instead; Black may reply with 8...h6, 8...Rb8, or 8...Be7.) White then  has a choice of retreats for the knight. The usual move here is 9.Nf3, after which Black obtains some initiative after 9...e4 10.Ne5 Bd6 (see diagram). This is the Knorre Variation, and is considered to be the main line of the Two Knights Defense. After ten moves, White has developed only two pieces against Black's three pieces and pawns, but has an extra pawn as well as a better pawn structure. Both 11.d4 and 11.f4 have been tried here with no definitive conclusion. 10...Bc5 is a viable alternative for Black, as is 10...Qc7 (the Goring Variation ). Steinitz favored 9.Nh3 instead, although it did not bring him success in his famous 1891 cable match against Chigorin. The Steinitz Variation was mostly forgotten until Fischer revived it in the 1960s. Nigel Short led a second revival of 9.Nh3 in the 1990s, and today it is thought to be about equal in strength to the more common 9.Nf3. In addition to the moves 8.Be2 and 8.Qf3, the move 8.Bd3 is a valid alternative that has apparently become fashionable in recent years. Traxler Variation: 4...Bc5. This bold move ignores White's attack on f7 and leads to wild play . Czech problemist Karel Traxler played it against Reinisch in Prague in 1890. Later it was named after Wilkes-Barre, Pennsylvania by Frank Marshall, who claimed to be first to analyze and publish it, so today 4...Bc5 is known as both the Traxler Variation and (in the United States and the United Kingdom only) the Wilkes-Barre Variation. White can play 5.d4, 5.Nxf7, or 5.Bxf7+. After 5.d4 d5!, White's best move is 6.Bxd5, reapplying the pressure on f7. 5.Nxf7 is very complicated after 5...Bxf2+. The current main lines all are thought to lead to drawn or  equal positions, e.g. after 6.Kxf2 Nxe4+ 7.Kg1, or even 7.Ke3. White's best try for an advantage is probably 5.Bxf7+ Ke7 6.Bb3 (although 6.Bd5 was the move recommended by Lawrence Trent), as this poses Black the most problems. No grandmasters have regularly adopted the Wilkes-Barre as Black, but Alexander Beliavsky and Alexei Shirov have played it occasionally even in top competition. No clear refutation is known. A tricky variation is 5.Bxf7+ Kf8!?, where Black plays for one last trick with 6.Bb3 d6 7.Nf7 Qe7. If White plays the seemingly standard 8.Nxh8??, Black is now winning after  8...Bg4!! 9.f3 Nxe4, making use of the pinned f3-pawn. This pawn cannot capture the bishop as 10.fxg4?? Qh4+ 11.g3 Bf2+ wins by force for Black. 4...Nxe4. 4...Nxe4?! is considered unsound but must be handled carefully. 5.Nxe4 d5 poses no problems for Black. If 5.Nxf7? Qh4! 6.g3 (6.0-0 Bc5!) 6...Qh3 7.Nxh8 Qg2 8.Rf1 Nd4 9.Qh5+ g6 10.Nxg6 hxg6 11.Qxg6+ Kd8 and Black has dangerous threats. (Alternatively, after 5.Nxf7? Qh4! 6.g3, Black could play more aggressively 6...Nxg3! 7.fxg3 Qe4+ 8.Qe2 Qxh1+ 9.Qf1 Qxf1+ 10.Kxf1 d5 11.Bxd5 Bh3 + 12.Ke1 Nb4 13.Bb3 Nxc2+ 14.Bxc2 Kxf7 with a distinct advantage of material for Black.) Correct is 5.Bxf7+! Ke7 6.d4! (6.d3 is also good) and now: 4.d4. White can choose to rapidly with 4.d4 exd4 5.0-0. Now Black can simply by eliminating White's last center pawn with 5...Nxe4, after which White regains the with 6.Re1 d5 7.Bxd5 Qxd5 8.Nc3, but Black has a comfortable position after 8...Qa5 or 8...Qh5. The Nakhmanson Gambit 6.Nc3 gives White compensation if Black accepts the piece with 6...dxc3 7.Bxf7+ Kxf7 8.Qd5+, but Victor Bologan suggests  6...Nxc3 7.bxc3 d5 8.Bb5 Be7 leading to a better position for Black. Alternatively, Black can enter the extensively analyzed Max Lange Attack after 5...Bc5 6.e5 d5, which can also arise by transposition from the Giuoco Piano or Scotch Game. White can choose to avoid these lines by playing 5.e5, a line often adopted by Sveshnikov. After 5.e5, either 5...Ne4 or 5...Ng4 is a reply, but most common and natural is 5...d5 6.Bb5 Ne4 7.Nxd4 Bc5, with play. The tricky 5.Ng5?! is best met by 5...d5! 6.exd5 Qe7+! 4.d3. The quiet move 4.d3 transposes  into the Giuoco Pianissimo if Black responds 4...Bc5, but there are also independent variations after 4...Be7 or 4...h6. White tries to avoid the tactical battles that are common in other lines of the Two Knights and to enter a more positional game. The resulting positions take on some characteristics of the Ruy Lopez if White plays c3 and retreats the bishop to c2 via Bc4–b3–c2. This move became popular in the 1980s and has been used by John Nunn and others. Four Knights Variation: 4.Nc3. The attempt to defend the pawn with 4.Nc3 does not  work well since Black can take the pawn anyway and use a fork trick to regain the piece, 4.Nc3?! Nxe4! 5.Nxe4 d5. The try 5.Bxf7+? does not help, as Black has the and a better position after 5...Kxf7 6.Nxe4 d5. Instead, 4.Nc3 is usually played with the intent to gambit the e-pawn with the Boden–Kieseritzky Gambit, 4.Nc3 Nxe4 5.0-0. This gambit is not commonly seen in tournament play as it is not well regarded by opening theory, but it can offer White good practical chances, especially in blitz chess. Chess opening theory table. White must  respond to the attack on the e-pawn. (For explanation of notation, see chess opening theory table.) References. Bibliography