The main alternative to the traditional move order for reaching the Semi-Slav is 1 d4 d5 2 c4 e6 3 Nc3 c6 4 Nf3 Nf6. The system where Black plays …d5, …e6 and …c6 prior to (or even without) …Nf6 is often referred to as the Triangle, so I refer to this move order as the Triangle move order.
Black’s primary motivation for choosing this order is to avoid the Exchange Variation of the Slav, which many people consider highly drawish. The point is that Black plays …e6 before White has the chance to play cxd5, so he can recapture with …exd5, with an asymmetrical pawn structure. However, while avoiding the Exchange Slav is undoubtebly a plus point, Black does give White some additional possibilities that can be tricky to handle.
The first alternative to consider is the aggressive Marshall Gambit (1 d4 d5 2 c4 e6 3 Nc3 c6 4 e4), which requires precise play from Black.
If White chooses 3 Nf3 rather than 3 Nc3, Black needs to be careful. If he plays 3…c6, then 4 Qc2 is a rather promising system; in contrast to 1 d4 d5 2 c4 c6 3 Nf3 Nf6 4 Qc2, Black cannot develop his queen’s bishop to f5 or g4, as he has already played …e6.
Black’s best reply to 3 Nf3 is 3…Nf6. White can then transposes to the Semi-Slav by 4 Nc3 c6, but he also has a major alternative in 4 g3, which leads play into the Catalan Opening. This is an option that does not exist in the traditional move order, as after 1 d4 d5 2 c4 c6 3 Nf3 Nf6 4 g3, Black can still develop his bishop to f5 or g4; this renders the Catalan formation ineffective.
Another challenging move after 3 Nf3 Nf6 is 4 Bg5. White hopes to trick Black into a standard Queen’s Gambit Declined after 4…Be7 5 Nc3, having sidestepped the Semi-Slav. This move order is poorly covered in chess literature; for instance, Larry Kaufman’s The Chess Advantage in Black and White fails to mention the move 4 Bg5 at all!
One solution to 4 Bg5 is the move 4…h6. If White then plays 5 Bxf6, then 5…Qxf6 6 Nc3 c6 transposes to a line of the Semi-Slav (1 d4 d5 2 c4 e6 3 Nc3 c6 4 Nf3 Nf6 5 Bg5 h6 6 Bxf6 Qxf6), while 5 Bh4 Bb4+ 6 Nc3 dxc4 is dubious for White. Of course, this is only viable if you meet 5 Bg5 with 5…h6.
Instead of 4…h6, Black can also consider 4…Bb4+ and 4…dxc4, which can both lead to complex play. For instance, 4…Bb4+ 5 Nc3 dxc4 6 e4 c5 is the sharp Vienna Variation.
Another alternative for White is the solid 4 e3. After 4…c5 5 Nc3 Nc6, play leads into the Symmetrical Variation of the Tarrasch. This offers chances to both sides.
White can also play 4 e3 after 1 d4 d5 2 c4 e6 3 Nc3 c6; in that case after 4…Nf6 5 Nf3, play will transpose into the Semi-Slav.
On move 2, if White plays 2 Nf3, Black should play 2…Nf6. Then 3 c4 e6 transposes back to the main line, while 3 e3 Bf5, 3 Bf4 c5 and 3 Bg5 Ne4 are all fine for Black.
White plays 1 c4
The response to 1 c4 that is consistent with the above system is 1…e6.
The lines 2 d4 d5, 2 Nc3 d5 3 d4 c6 and 2 Nf3 d5 3 d4 Nf6 all transpose to systems covered above.
2 e4 d5 leads to a sideline of the French Defence. 3 exd5 exd5 4 d4 is an aggressive line of the French Exchange.
If White holds back both d4 and e4, play usually leads into the Reti Opening.
White plays 1 Nf3
Black can meet 1 Nf3 with 1…d5, when 2 d4 Nf6 transposes to the lines covered after 1 d4, while 2 c4 e6 transposes to the lines covered after 1 c4.
Should White continue with 2 g3 or 2 b3, Black can develop solidly, looking to develop his bishop on f5 or g4. If White plays c4, play will lead to lines of the Reti Opening.