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This article uses algebraic notation to describe chess moves.

The Four Knights Game is a chess opening starting with the moves

1.e4 e5

2.Nf3 Nc6

3.Nc3 Nf6

This is the most common sequence, but the knights may be developed in any order. The ECO codes for the Four Knights Game are C47 (anything but 4.Bb5), C48 (4.Bb5 without 4...Bb4), and C49 (the Symmetrical Variation, 4.Bb5 Bb4).

The Four Knights is fairly popular with beginners who strictly adhere to the opening principle "develop knights before bishops." It is occasionally played at higher levels, but most players consider the Ruy Lopez a better try for advantage. The Four Knights usually leads to quiet positional play but there are a few fairly sharp variations.

White's most common move is 4.Bb5, after which Black has three major alternatives. 4...Bb4 is the Symmetrical Variation. Often it results in a quick draw, but it is possible for either side to play for a win. Black can play more aggressively by 4...Nd4, the Rubinstein Variation. White cannot win a pawn with 5.Nxe5, since Black regains the pawn with the advantage of the bishop-pair after 5...Qe7 6.Nf3 (6.f4 Nxb5 7.Nxb5 d6) Nxb5 7.Nxb5 Qxe4+ 8.Qe2 Qxe2+ 9.Kxe2 Nd5! 10.c4 a6! White most often plays 5.Ba4, when Black usually continues in gambit fashion with 5...Bc5!? 6.Nxe5 0-0 7.Nd3 Bb6 8.e5 Ne8 followed by ...d6. Another line, which discourages many ambitious Black players from playing the Rubinstein, is 5.Nxd4 exd4 6.e5 dxc3 7.exf6 Qxf6 (7...cxd2+?! 8.Bxd2 Qxf6 9.0-0 is dangerous for Black) 8.dxc3 Qe5+. This often leads to a quick draw after 9.Qe2 Qxe2+. In recent years, Black has tried 4...Bd6!? with success. That move takes the sting out of 5.Bxc6, which is met with 5...dxc6 with a good game. If White plays quietly, Black will regroup with ...0-0, ...Re8, ...Bf8, and ...d6.

If White plays 4.d4, the Scotch Four Knights Game arises. This leads to a more open position, which can also be reached from the Scotch Game, e.g. 1.e4 e5 2.Nf3 Nc6 3.d4 (the Scotch Game) exd4 4.Nxd4 Nf6 5.Nc3. This variation was played in the fifth game of the 1996 Deep Blue vs. Garry Kasparov match, and was the only game between the two where Black won.

One reason White may choose the Four Knights (3.Nc3) move order over the Scotch (3.d4), besides fearing that after 3.d4 ed4 4. Nxd4 Black may choose 4...Bc5 or 4...Qh4, is that White may want to play the Belgrade Gambit (1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.d4 exd4 5.Nd5). It is not possible to reach the Belgrade from the Scotch. However, the Belgrade is a distant second in popularity to 5.Nxd4.

A further possibility is 4.Bc4 (the Italian Four Knights). Black can either preserve the symmetry by 4...Bc5, leading to the quiet Giuoco Pianissimo, or pseudo-sacrifice a knight with 4...Nxe4!, the center fork trick. If 5.Nxe4, then 5...d5 regains the piece. Bxf7+?, though superficially attractive, just trades bishop and center pawn for a knight and side pawn, with the initiative soon passing to Black.

The quiet waiting move 4.a3 is a specialty of Polish Grandmaster Pawel Blehm. White gets no advantage after 4...d5 (a Scotch Four Knights Game Reversed).

A dubious but dangerous gambit is the so-called Halloween Gambit, 4.Nxe5?!. After 4...Nxe5 5.d4, White tries to seize the center with his pawns and drive the Black knights back to their home squares. International Master Larry Kaufman, in his 2004 book The Chess Advantage in Black and White (p. 328), says that this line is refuted by 5...Nc6 6.d5 Bb4! 7.dxc6 Nxe4 8.Qd4 Qe7, which he attributes to Jan Pinski.

Similar sacrifices from Black can arise if White plays the Glek Variation, 4.g3. Black can now make a Halloween-type sacrifice with 4...Nxe4. This is probably more sound than White's Halloween sacrifice since 4.g3 has weakened the important f3 square and robbed the g3 square from the White pieces. This line has been tried by Magnus Carlsen. After 5.Nxe4 d5 6.Nc3 d4, White entered a variation of the Vienna Game by returning the piece with 7.Bg2 rather than fight against prepared analysis.

• Tim Krabbé's article on the Halloween Gambit