Two Knights Defense, Open Variation

The Open Variation, 4. d4, is the most direct of White's replies to the Two Knights Defense. Instead of covering e4 with d3 or Nc3, or going after f7 with Ng5, White hits the e5-pawn immediately. This clears the diagonal for the c1-bishop and leaves the e4-pawn hanging. Black has several ways to resolve the tension, but only some of them are sound. The main reply, 4...exd4, reaches a Scotch Gambit position, so the variation works largely as an entrance into that complex.

About two-thirds of players answer with 4...exd4. One option is 5. e5, gaining space and driving the f6-knight away. Black can put it on e4 at once with 5...Ne4, or insert 5...d5 first. After 6. Bb5 Ne4 the d-pawn supports the advanced knight, and the pawn move has gained time by hitting the c4-bishop. White can also play 5. Ng5, the Perreux Variation, or castle first with 5. O-O Bc5 6. e5, which is the Max Lange Attack.

4...d6, the Scotch Invitation Declined, holds e5 and stays out of gambit territory. White can liquidate with 5. dxe5 dxe5 6. Qxd8+ Nxd8 7. Nxe5 Nxe4, leading to a queenless middlegame. The alternative 5. d5 gains space and pushes the knight back from c6.

The captures on e4 and d4 are considered inferior. After 4...Nxe4 5. dxe5, White threatens both Bxf7+ and Qd5, which would hit the knight and f7 together. The natural 5...Bc5 loses material to 6. Qd5 Bxf2+ 7. Ke2 O-O 8. Qxe4, when the bishop on f2 is left loose. The more resilient 5...d6 prepares ...Be6, but 6. Bxf7+ Kxf7 7. Qd5+ Be6 8. Qxe4 still leaves White a pawn ahead. Taking the other pawn with 4...Nxd4 runs into 5. Bxf7+ Kxf7 6. Nxe5+ Ke8 7. Qxd4. White has an extra pawn and better development, and Black can no longer castle. Even so, 7...Qe7 with the idea ...c5 is a practical resource: after the careless 8. O-O c5 9. Qc3 Nxe4, Black has equalized.

The engine considers the position after 4. d4 roughly balanced. In online games White wins somewhat more often than Black, and draws are very rare. Of the main replies, 4...Nxe4 gives Black the poorest results.