Two Knights Defense, Polerio Defense, Kieseritzky Variation

The Kieseritzky Variation, also called the Morphy Variation, has White reply to 5...Na5 with 6. d3 instead of checking on b5. White keeps the d5-pawn and lets Black exchange the knight for the c4-bishop. After ...Nxc4 the recapture dxc4 undoubles the d-pawns. The cost is the Italian bishop, given up for a knight that was out of play on a5. This is the main reason the line is far less common than 6. Bb5+. The engine gives Black a slight edge, though online results are almost exactly even, with few draws.

Most often Black takes at once with 6...Nxc4. The alternative 6...h6 scores better for Black: it gains time on the g5-knight before the bishop is captured. A typical line is 6...h6 7. Nf3 e4 8. Qe2, pinning the e-pawn. Then 8...Nxc4 9. dxc4 Bc5 10. Nfd2 reaches the Yankovich Variation, where White targets the advanced e4-pawn. The recapture 6...Nxd5 is rare and scores poorly for Black, since it releases the pressure the knight move was meant to create.