Semi-Slav Defense

The Semi-Slav Defense arises after 4...e6, when Black's pawns on c6, d5 and e6 blend the Slav Defense with the Orthodox Queen's Gambit Declined. The Encyclopaedia of Chess Openings files it under D43 to D49. Black deliberately shuts the light-squared bishop in behind the chain and plans to release it later with ...e5 or ...c5. The concrete threat is ...dxc4 followed by ...b5, holding the extra pawn, so White's fifth move must address it. The position is reached through many move orders, including from the Queen's Gambit Declined via 1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Nf3 c6.

5. Bg5, chosen in more than half of games, produces sharp and tactical positions. Black can take the pawn with 5...dxc4, the Botvinnik Variation, also catalogued as the Semi-Slav Accepted, after which White plays a genuine pawn sacrifice for activity. Another option is 5...h6, and after 6. Bh4 the line is known as the Anti-Moscow Gambit.

5. e3, the Main Line, is calmer and more positional. It protects c4 at the cost of leaving the dark-squared bishop inside the pawn chain. Black usually answers 5...Nbd7, the Normal Variation. After 6. Bd3 dxc4 7. Bxc4 b5, the Meran Variation, Black gives up the center in return for queenside space and a tempo on the bishop, while White aims for e3-e4. The most common retreat is 8. Bd3, and Black then chooses between 8...a6 (Classical Meran) and 8...Bb7 (Modern Meran). The Meran first appeared in Schlechter–Perlis in 1906. It is named after the Meran tournament of 1924, where Grünfeld won with it against Rubinstein. Before the Meran became standard, 6...Bd6, the Chigorin Defense, was the main line, and Arthur Bisguier played it throughout his career. White can also sidestep the Meran with one of several Anti-Meran systems.

White has quieter fifth moves. 5. cxd5 exd5 leads to an Exchange Queen's Gambit Declined in which the early Nf3 lets Black's queen's bishop develop freely, and equality is expected. 5. g3 avoids heavy theory and can lead to Catalan-like positions. 5. Bf4 is the third most popular choice on Lichess, but 5...dxc4 is regarded as good for Black.

Black can also use the triangle order 1. d4 d5 2. c4 e6 3. Nc3 c6, the Accelerated Move Order. This allows the Marshall Gambit with 4. e4. After 4. Nf3 it adds the Noteboom Variation, 4...dxc4, and 4. e3 f5 gives the Anti-Noteboom Stonewall Variation. The engine rates the starting position as roughly balanced, and White scores somewhat better in Lichess games.