1...d5
This is one of Black's best replies, occupying the center and undermining the unsettled position of White's knight. White can prevent 2...d4 by playing 2.d4 themselves, transposing to a well-known position in the Queen's Pawn Game, where after 2...Nf6 white can choose to play a Jobava London system with 3.Bf4, a Veresov attack with 3.Bg5 or he may transpose to the Blackmar-Deimer Gambit with 3.e4?!.
Also possible is 2.Nf3 (and if 2...d4, 3.Ne4), a sort of Black Knights' Tango with an extra move ( Harding 1974 :10). A third line is 2.e3, which Keilhack calls "the Müller game," when White hopes for 2...e5 (other moves are also playable, of course) 3.Qh5!?, e.g. 3...Nc6 4.Bb5 Qd6 5.d4 exd4 6.exd4 Nf6 7.Qe5+! Be6?! (Keilhack recommends 7...Kd8!! 8.Bxc6 bxc6 9.Nf3 Bg4) 8.Bf4 0-0-0 9.Bxc6 Qxc6? 10.Nb5!, when White wins at least a pawn ( Keilhack 2005 :307–11).
White's most common response to 1...d5 is 2.e4. This is the same position as 1.e4 d5 2.Nc3, an obscure branch of the Scandinavian Defense. Black has five plausible responses to 2.e4: 2...e6 and 2...c6 transpose to the French and Caro–Kann Defenses, and 2...Nf6 to a variation of Alekhine's Defence. The move 2...d4 gives Black a spatial advantage, which White may work to undermine along the lines of hypermodernism. Keilhack writes, "2...d4 is chosen either by somewhat naive players who are attracted by the fact that Black wins time and space ... or by strong players who are aware of the strategic risks but are striving for a complex battle." ( Keilhack 2005 :44) He considers the Van Geet Attack, 2...d4 3.Nce2 followed by 4.Ng3, to be "the core of the 1.Nc3 opening," "a fully independent entity which strives for early knight activity on the kingside," usually with Ng3, Nf3, Bc4 or Bb5, 0-0, and d3 ( Keilhack 2005 :44). Alternative lines for White include the unusual 3.Nb1!?, with which van Geet once drew Spassky, and, after 3.Nce2, playing a sort of King's Indian Attack with d3, g3, Bg2, f4, Nf3, and 0-0 – a line Keilhack calls the "Lizard Attack" ( Keilhack 2005 :125).
The fifth alternative, 1...d5 2.e4 dxe4, leads to more open play. After 3.Nxe4, Black has a number of playable moves, including 3...e5, 3...Nc6, 3...Bf5, 3...Nd7, 3...Nf6, and even 3...Qd5!?, when 4.Nc3 transposes to the Scandinavian Defense ( Keilhack 2005 :131, 144, 146, 158, 172, 176). After 3...e5, White's thematic move is 4.Bc4, when several of Black's plausible moves lead to disaster, e.g. 4...Be7? 5.Qh5! and White wins at least a pawn after 5...g6 6.Qxe5 or 5...Nh6 6.d3; or 4...Nf6? 5.Ng5! Nd5 and now 6.d4!, 6.Qf3!, and 6.Nxf7!? Kxf7 7.Qf3+ are all possible, with positions similar to the line of the Two Knights Defense beginning 1.e4 e5 2.Nf3 Nc6 3.Bc4 Nf6 4.Ng5 d5 5.exd5 Nxd5?! ( Keilhack 2005 :133–34). However, 3...e5 4.Bc4 Nc6! is playable ( Keilhack 2005 :135–43).
International Master Richard Palliser, in his 2006 book Beating Unusual Chess Openings, recommends 1...d5 2.e4 dxe4 3.Nxe4 Nd7 for Black. He explains, "Black doesn't attempt to refute White's opening or to gain lots of space (as with 2...d4), but simply settles for sensible development. The position should be compared to both a Caro-Kann Defense and a French Rubinstein. Black will hope to demonstrate that he has gained from the omission of an early ...c6 or ...e6, while White will generally omit d4, preferring a setup with Bc4 and d3" ( Palliser 2006 :143). After 4.Bc4, the natural move 4...Ngf6!? leads to very sharp and unclear play if White responds with 5.Bxf7+!? Kxf7 6.Ng5+ Kg8 7.Ne6 Qe8 8.Nxc7 ( Keilhack 2005 :158–63)( Palliser 2006 :144–46). More solid is 4...e6 ("!" – Keilhack) ( Keilhack 2005 :164–70)( Palliser 2006 :144–48).
1...c5
1...c5 is often played by devotees of the Sicilian Defence, into which the game often transposes, either immediately after 2.e4 or at a later point. Alternatively, White can remain in independent 1.Nc3 lines, at least for the time being, with 2.Nf3 followed by 3.d4, which gives Black a large choice of possible responses. One line Palliser recommends for Black is 2.Nf3 Nf6 3.d4 (3.e4 transposes to a Sicilian) cxd4 4.Nxd4 d5!? (seizing the center) 5.Bg5 Nbd7! 6.e4 (more critical than the passive 6.e3?!) dxe4 7.Qe2 e6!? 8.0-0-0 Be7 9.Nxe4 0-0 when "White doesn't appear to have any advantage" ( Palliser 2006 :154–56).
If White chooses to transpose to standard Sicilian lines, the fact that his knight is committed to c3 may be a disadvantage in certain lines. The Closed Sicilian, commonly reached by 1.e4 c5 2.Nc3, without an early d4 by White, gives Black few theoretical difficulties ( de Firmian 2008 :346). If White instead chooses to play an Open Sicilian with 2.e4 and 3.Nf3 or Nge2, followed by d4, the knight's placement on c3 prevents White from playing the Maróczy Bind with c4. This makes the Accelerated Dragon Variation with 2...Nc6 and 3...g6 particularly attractive ( Gallagher 1994 :146). Black may also stop White's intended d4 by playing an early ...e5, e.g. 1.Nc3 c5 2.e4 Nc6 3.Nf3 e5 ( Gallagher 1994 :151–55) or 3.Nge2 e5 ( Gallagher 1994 :148–51).
1...Nf6
Grandmaster Larry Kaufman recommends 1...Nf6, intending to meet 2.e4 with 2...e5 or 2.d4 with 2...d5 ( Kaufman 2004 :484). Keilhack writes that "1...Nf6 is one of the most unpleasant replies for the 1.Nc3 player. Black keeps all options open, he can choose between a central (...d5, possibly followed by ...c5) and an Indian (...g6, ...Bg7) setup. ... Among the many possible [second] moves [for White], none really stands out." ( Keilhack 2005 :338). The most straightforward moves for White are 2.d4 and 2.e4, but neither promises White a significant advantage. After 2.d4, 2...d5 leads to the Richter-Veresov Attack (3.Bg5) or another type of Queen's Pawn Game such as a Jobava London System, where White, having blocked his c-pawn, has little chance for an advantage ( Kaufman 2004 :469, 473). After 2.e4, Black can again play 2...d5 with a variation of the Alekhine's Defense; or 2...d6 3.d4 g6 with a Pirc Defense or 3...e5 with a Philidor's Defense. The most solid response to 2.e4 is 2...e5, leading to a Vienna Game or, after 3.Nf3 Nc6, to a Four Knights Game—neither of which offers White an appreciable advantage ( Kaufman 2004 :364–65)( de Firmian 2008 :121). Keilhack also analyzes a number of offbeat possibilities, including 2.b3, 2.Nf3, 2.f4 (an unusual form of Bird's Opening that Keilhack calls the "Aasum System"), 2.g3, and even the gambit 2.g4?! Palliser writes that none of the alternatives to 2.e4 "really convince or should greatly trouble Black over the board" ( Palliser 2006 :142).
1...e5
This natural move is probably playable, but already a slight inaccuracy as White maintains an opening advantage in all lines. It is also particularly dangerous if Black does not know what he is doing, with numerous traps and knight tactics which Black must avoid. One of the main lines is 1...e5 2.Nf3 Nc6 3.d4, known as the Napoleon Attack, with poor chances for Black, continued by 3...exd4 4.Nxd4 Nf6 5.Bg5. Keilhack writes that this variation "occurs rather often and offers excellent chances for an early knockout" by White and that "only two [moves] (5...Bb4 and 5...Bc5) do not immediately ruin Black's game" ( Keilhack 2005 :26). (See, e.g., the Dunst–Gresser game given below.)
Additionally, White can opt to immediately transpose into mainstream opening territory with 2.e4, leading the game into King's Pawn Openings such as the Vienna Game, or Four Knights Game, however, it is thought that 2.Nf3 followed by 3.d4 is White's best chance of gaining an advantage out of the opening.
