Two Knights Attack

After 4...dxc4, often called the pure Slav or Slav Accepted, Black takes the c4-pawn and threatens to keep it with ...b5, supported by the pawn on c6. The line is roughly half as common as 4...e6, the Semi-Slav. White must decide whether to prevent the queenside expansion or let it happen and play for development and the center.

5. a4, the Alapin Variation, is the most frequent reply and the main line. It rules out ...b5 but spends time recovering the pawn, which gives Black the moment to develop the queen's bishop. The gambit approaches give the pawn up. 5. e4, the Geller Gambit (also called the Slav Gambit), builds a broad pawn center and allows Black to advance on the queenside; its main line is considered good for Black, though less explored branches remain dangerous. 5. e3, the Alekhine Variation, is more modest and keeps White's center intact, aiming to induce overextended queenside pawns and undermine them later. 5. Bg5 is also common, while 5. g3 and 5. Ne5, the latter preparing a fianchetto against the long diagonal, are rarer. Engines rate the position close to balanced.