Bonsch-Osmolovsky Variation

The Bonsch-Osmolovsky Variation is a prepared form of the Modern Defense. Instead of striking with 3...d5 immediately, Black first develops the king's knight to e7, so that after the central break the knight can recapture on d5. From there it covers f4, letting Black keep the gambit pawn without committing to ...g5. The typical sequence is 4. d4 d5, and if White exchanges with 5. exd5, then 5...Nxd5 gives Black a centralized knight guarding f4. White is not obliged to cooperate, however. After 4. d4 d5, the reply 5. Nc3 maintains the e4-pawn and keeps the tension in the center.

The most frequent reply to 3...Ne7 is 4. Bc4, aimed at f7, played slightly more often than 4. d4. The rarer 4. Nc3 also adds support to e4. Engines regard the position as roughly balanced, and in online games Black scores slightly better than White. It is a little-explored sideline compared with the main defenses after 3. Nf3.