Main Line, Slav Accepted: 3.Nf3 Nf6 4.Nc3 Dxc4
The main line continues 3.Nf3 Nf6.
A line that is similar to the Catalan Opening is 4. Qc2. Often, White will fianchetto his Light Square Bishop. This has the disadvantage of White's Queen being somewhat exposed on c2. Black can continue 4...g6, intending 5...Bf5. White usually plays 5.Bf4 so that after 5...Bf5 6. Qb3 Qb6 White can play 7. c5! Black has to play 7...Qxb3, which will be met by 8.axb3. White has a moderate advantage in this Queenless middlegame, as white can expand on the queenside and try to create on the queenside, but black's position is solid.
The main line continues 4. Nc3.
Black shouldn't play 4...Bf5 because White will gain the advantage with either 5.Qb3 or 5.cxd5 followed by 6.Qb3. Traditionally Black had a choice between 4...e6 and 4...dxc4 before developing the queen bishop, but in the 1990s 4...a6 was introduced, with the idea of developing the queenside without locking in the queen bishop or conceding the center. As before, 4...e6 transposes into the Semi-Slav.
The Slav Accepted continues with 4...dxc4.
Read more about this topic: Slav Defense
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