Karpov Variation

The Karpov Variation, 4...Nd7, is also known as the Steinitz, Smyslov or Modern Variation. Wilhelm Steinitz played it, and the line is also named after him. The knight move prepares ...Ngf6, so that if White exchanges on f6 Black can recapture with the other knight and keep an intact pawn structure. Compared with 4...Bf5, the c8-bishop is not yet committed and need not end up on g6. The cost is time. The bishop stays inside until the d7-knight moves again, and White has more room to take space in the center. Black has to watch for the trap 5. Qe2 Ngf6?? 6. Nd6#. After 5...Ndf6 instead, Black's position is sound.

5. Nf3 is played in about half of games, and 5...Ngf6 6. Nxf6+ Nxf6 is a typical continuation. The aggressive 5. Ng5 is the Modern Variation. Its main line runs 5...Ngf6 6. Bd3 e6 7. N1f3 Bd6 8. Qe2 h6 9. Ne4 Nxe4 10. Qxe4. The last game of the Deep Blue–Kasparov rematch was played in this line. After 5. Bc4 Ngf6, White can play 6. Nxf6+ Nxf6, the Tiviakov-Fischer Attack, or 6. Ng5 e6 7. Qe2 Nb6, the Smyslov Variation. Engines give White a slight edge, and results are close to even.