Two Knights Defense, Polerio Defense

The Polerio Defense, 5...Na5, is Black's standard reply to 4. Ng5 d5 5. exd5 in the Two Knights Defense. Instead of recapturing on d5, which runs into the Fried Liver and Lolli attacks, Black moves the attacked c6-knight to a square where it hits the bishop on c4. The d5-pawn stays, so White is a pawn up for now. In return White's d-pawns are doubled, the knight on g5 is exposed, and the d- and e-files are half-open. The engine rates the position as roughly balanced, and Black scores slightly better than White in online games, with very few draws.

The main reply, played in about three games out of four, is 6. Bb5+, the Bishop Check Line. After 6...c6 7. dxc6 bxc6 the doubled pawn is gone and White's extra pawn is a healthy one. Black, though, has an open b-file, a lead in development, and the idea of ...h6 and ...e4 to gain time against the white knight. The bishop must now retreat. 8. Qf3, the Bogoljubow Variation, pins the c6-pawn against the rook on a8 and keeps the bishop on b5 for the moment. The more common retreat 8. Be2 is met by 8...h6, the Suhle Defense, which drives the knight back. After 9. Nf3 e4 10. Ne5 the knight takes a central post, and 10...Qc7 gives the Göring Variation. In these lines Black aims to show that the space and activity are worth the pawn.

With 6. d3, the Kieseritzky Variation, White does not check and lets the bishop be exchanged, since recapturing with the d3-pawn on c4 undoubles the pawns. A typical continuation is 6...h6 7. Nf3 e4 8. Qe2, pinning the e-pawn. After 8...Nxc4 9. dxc4 Bc5 10. Nfd2 the position is the Yankovich Variation, where White tries to win the advanced e4-pawn.

The retreat 6. Bb3 is played about as often as 6. d3 but scores poorly for White. It allows ...Nxb3, which takes the bishop pair without any gain in structure. In every branch the play is concrete: Black's compensation depends on tempo, so accurate move-by-move play matters more than general plans. The line takes its name from Giulio Polerio, the 16th-century analyst of the Italian Game.