Alapin Variation

After 4...dxc4, White's 5. a4 rules out ...b5, so Black cannot hold the extra pawn and White can count on recovering it with e3 or e4 and Bxc4. It is White's most popular and most ambitious answer to the pure Slav. The price is a permanently weakened b4-square, which Black can use as a post for a knight or a bishop. Engines give White a slight edge, and White scores a little better in the database.

Black's four principal replies differ mainly in where the queen's bishop goes. 5...Bf5, the Czech Variation, is by far the most common and the most heavily analyzed. It develops the bishop actively and makes e4 harder to achieve. 5...Bg4, the Steiner Variation, is popular at amateur level but demands accurate understanding. Because the knight on f3 is not truly pinned, White can play Ne5 and gain time on the bishop. 5...e6, the Soultanbeieff Variation, gives the game a Semi-Slav character and leaves the bishop temporarily inside the pawn chain. 5...Na6, the Smyslov Variation, sends the knight toward b4, where no pawn can drive it away.