Editing Two Knights Defense (section)

====Main line: 4...d5 <span class="anchor" id="Main line"></span>====
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:
* The recapture 5...Nxd5{{chesspunc|?!}} is extremely risky. [[Albert Pinkus]] tried to bolster this move with analysis in 1943 and 1944 issues of ''[[Chess Review]]'', but White gets a strong attack with either the safe [[Giambattista Lolli|Lolli]] Attack 6.d4{{chesspunc|!}}, which [[Bobby Fischer]] thought to be very strong,<ref name=My60MG>{{cite book|last=Fischer|first=Bobby|author-link=Bobby Fischer|title=[[My 60 Memorable Games]]}} Fischer's analysis of this line is in game 45, Fischer–Bisguier, 1963</ref> or the sacrificial [[Fried Liver Attack|Fried Liver (or Fegatello) Attack]] 6.Nxf7! Kxf7 7.Qf3+ Ke6 8.Nc3. These variations are usually considered too difficult for Black to defend {{chessgloss|over the board}}, but they are sometimes used in correspondence play. Lawrence Trent describes 5...Nxd5 as "a well-known bad move" (or words to that effect).<ref name=LT/>
* The Fritz Variation 5...Nd4 and Ulvestad's Variation 5...b5 are related as they share a common subvariation. American master Olav Ulvestad introduced 5...b5 in a 1941 article in ''Chess Review''. White has only one good reply: 6.Bf1!, protecting g2 so White can answer 6...Qxd5{{chesspunc|?}} with 7.Nc3. Both replies 6.Bxb5 Qxd5 7.Bxc6+ Qxc6 and 6.dxc6 bxc4 7.Nc3 are weak for White. Black's best response is to [[Transposition (chess)|transpose]] to the Fritz Variation with 6...Nd4, making another advantage of 6.Bf1 apparent; the bishop is not attacked as it would be if White had played 6.Be2. German master [[Alexander Fritz]] (1857–1932) suggested 5...Nd4 to [[Carl Schlechter]], who wrote about the idea in a 1904 issue of ''[[Deutsche Schachzeitung]]''. In 1907 Fritz himself wrote an article about his move in the Swedish journal ''[[Tidskrift för Schack]]''. White's best reply is 6.c3, when the game often continues 6...b5 7.Bf1 Nxd5 8.Ne4 or 8.h4.
* The Berliner Variation, named in honor of IM / GMC Dr. [[Hans Berliner]], continues the Fritz sub-line 8.Ne4 with 8...Qh4, from the famous game Estrin–Berliner, World Correspondence Championship 1965–68, eventually won by Black; this win was pivotal to Berliner's eventual championship victory. That game, which saw Black embark on a very sharp sacrificial path, continued 9.Ng3 Bg4 10.f3 e4 11.cxd4 Bd6 12.Bxb5+ Kd8 13.0-0 exf3. IM / GMC Estrin later published, in 1971, the suggestion 14.Qb3!? as an improvement on the game continuation, and this possibility has continued to interest many players.<ref>{{cite book |title=Nunn's Chess Openings |publisher=[[Everyman Chess]] |year=1999 |location=London |at=section on Two Knights' Defense}}</ref>

{{Chess diagram
|tright
|
|rd|  |bd|qd|kd|  |  |rd
|pd|  |  |  |  |pd|pd| 
|  |  |pd|bd|  |nd|  |pd
|nd|  |  |  |nl|  |  | 
|  |  |  |  |pd|  |  | 
|  |  |  |  |  |  |  | 
|pl|pl|pl|pl|bl|pl|pl|pl
|rl|nl|bl|ql|kl|  |  |rl
|Main line after 10...Bd6
}}
After 5...Na5, the Polerio Defense,<ref>{{Cite web |title=Italian Game: Knight Attack, Polerio Defense – Chess Openings |url=https://www.chess.com/openings/Italian-Game-Knight-Attack-Polerio-Defense |access-date=2022-07-26 |website=Chess.com |language=en-US |archive-date=2023-01-05 |archive-url=https://web.archive.org/web/20230105035300/https://www.chess.com/openings/Italian-Game-Knight-Attack-Polerio-Defense |url-status=live }}</ref><ref>{{Cite web |title=POLERIO DEFENSE |url=https://www.chessgames.com/perl/chesscollection?cid=1041208 |access-date=2022-07-26 |website=www.chessgames.com |archive-date=2022-07-26 |archive-url=https://web.archive.org/web/20220726063003/https://www.chessgames.com/perl/chesscollection?cid=1041208 |url-status=live }}</ref> [[Paul Morphy]] would play to hold the gambit pawn with 6.d3, the [[Lionel Kieseritzky|Kieseritzky]] Attack (or Morphy Variation), which 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.<ref name="Bronstein"/><ref>{{cite web |url=http://www.chessgames.com/perl/chessgame?gid=1033954 |title=Bronstein vs. Rojahn, Moscow Olympiad 1956 |website=[[Chessgames.com]] |access-date=2005-11-19 |archive-date=2007-12-15 |archive-url=https://web.archive.org/web/20071215075353/http://www.chessgames.com/perl/chessgame?gid=1033954 |url-status=live }}</ref>)

Instead, White usually plays 6.Bb5+, when play usually continues 6...c6 (6...Bd7 is also possible<ref name=LT/>) 7.dxc6 bxc6 8.Be2 h6. (The move 8.Qf3, popular in the nineteenth century and revived by [[Efim Bogoljubow]] 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).<ref>[http://www.ajedrezaranjuez.com/openings.aspx?cod=C50-C59 Chess Openings Viewer] {{Webarchive|url=https://web.archive.org/web/20160304195212/http://www.ajedrezaranjuez.com/openings.aspx?cod=C50-C59 |date=2016-03-04 }}, C59: Club Aranjuez de Ajedrez</ref>

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.<ref name=LT/>