Accelerated Move Order

The Accelerated Move Order arises when Black, after 1. d4 d5 2. c4 e6 3. Nc3, answers with 3...c6 and postpones ...Nf6. The pawns on c6, d5 and e6 form the shape that gives the line its other common name, the Triangle system. Keeping the knight at home leaves Black several paths: a later ...Nf6 returns to the Queen's Gambit Declined or the Semi-Slav proper, while other moves lead to independent setups. The drawback is that e4 is no longer covered by a knight, and White can strike there immediately.

4. Nf3 is the usual reply, chosen in a little under half of games. Besides transposing with 4...Nf6, Black can take on c4 at once with 4...dxc4, the Noteboom Variation. Black hangs on to the extra pawn and aims to create two passed pawns on the queenside, conceding White a broad pawn center. The middlegame is dangerous for Black, but an endgame reached safely tends to favor the side with the queenside majority.

4. e3, the engine's top choice, closes the c1-h6 diagonal but rules out the Noteboom. Black can still transpose with 4...Nf6, or switch to 4...f5, the Anti-Noteboom Stonewall Variation. With the dark-square grip on e4, Black typically turns to the kingside, often bringing a rook across via f6 to join an attack.

4. e4, the Marshall Gambit, exploits the missing knight most directly. The principal line runs 4...dxe4 5. Nxe4 Bb4+ 6. Bd2 Qxd4 7. Bxb4 Qxe4+, and White normally interposes Be2, allowing Black's queen to take on g2. The rook on h1 remains taboo, because Bf3 would trap the queen. For the pawns White obtains a long-lasting initiative, with the dark squares around d6 a recurring tactical target; Bacrot–Morozevich, Biel 2012, is a known example. On Lichess the gambit gives White the best results of the main fourth moves.

4. cxd5 and 4. Bf4 together account for roughly a quarter of games and avoid the most forcing theory. The position ranks among the more frequently reached named openings, the engine considers it roughly balanced, and White scores slightly better in practice.