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The Semi-Slav Defense is a variation of the Queen's Gambit chess opening, defined by the opening moves (in algebraic notation 1.d4 d5 2.c4 e6 3.Nc3 Nf6 4.Nf3 c6, although the moves are not always played in that order. The opening resembles a mixture of the Orthodox Queen's Gambit Declined and the Slav Defense.

With 4...c6 Black is threatening to take the white pawn on c4, and keep it with a ...b5 advance. White must decide whether to allow this, gambiting a pawn, in exchange for the initiative or hold on to it with 5.e3, at the cost of blocking the dark-squared bishop.

In the Encyclopaedia of Chess Openings, the Semi-Slav has codes D43 to D49.

Main Variations

• The main variation of the Semi-Slav is the Meran Variation, 5.e3 Nbd7 6.Bd3 dxc4, (ECO codes D46 to D49), when play usually continues with 7.Bxc4 b5 8.Bd3. Black surrendered the centre for a moment, but won a tempo because the white bishop moved twice, first to d3 then to c4. Black then made a grab for queenside space and initiative by playing ...b5. White will play in the center, Black will aim to get a powerful initiative on the queenside often leading to a tense, sharp and complicated game.

• The Anti-Meran Gambit 5.Bg5, ECO code D44, is also extremely complicated. White refuses to shut in the dark-squared bishop, instead developing it to an active square where it pins the black knight, but Black can now win a pawn by 5...dxc4. White takes charge of the centre with 6.e4 and Black proceeds to defend his booty with 6...b5. Usually the game now continues with 7.e5 h6 8.Bh4 g5 9.Nxg5 hxg5 10.Bxg5 Nbd7. White will get his piece back, along with two pawns, meaning he will be a pawn up, but Black will soon complete his development. This is frequently referred to as the Botvinnik System. White will fianchetto his king's bishop and castle kingside, while black will play ...c5, ...Qb6, and castle long, leading to absurdly complex play.

• The Moscow Variation 5.Bg5 h6 6.Bxf6 (the gambit line 6.Bh4!? is rarely seen today) Qxf6, leads to double-edged play. Black has the bishop pair, but must avoid allowing White to open the game too quickly and exploit Black's lagging development.

• Calmer lines can be found in the remaining ECO codes, D43 and D45. If White wants to avoid the Meran Variation without entering the muddy waters of the Anti-Meran gambit, 5.cxd5 or 5.Qb3 are possibilities. Alternatively, after 5.e3 Nbd7, a popular alternative to 6.Bd3 is 6.Qc2.