Classical Variation

The Classical Variation, 4...Bc5, develops the bishop with an immediate threat to the knight on d4, which only the queen protects at that moment. The bishop also aims at f2. White has to resolve the attack by exchanging the knight, reinforcing it or moving it away, and each of these choices gives the game a different character. The engine considers the position roughly balanced. Lichess games from here are rarely drawn, and White's results are only slightly better than Black's.

5. Nxc6 is the most frequent reply and gains time against the black queen. Recapturing with 5...dxc6 allows a queen exchange that costs Black the right to castle. The usual answer is therefore the Intermezzo Variation, 5...Qf6, which threatens 6...Qxf2# and lets Black take back on c6 after White has dealt with the threat. This branch produces almost identical results for both sides.

5. Be3 protects the knight and blocks the bishop's diagonal toward f2. It also sets a trap: 5...Nf6?? loses a piece to 6. Nxc6 dxc6 7. Qxd8+ Kxd8 8. Bxc5. The main reply is 5...Qf6, which attacks d4 a second time, and White supports the knight with 6. c3. The traditional continuation 6...Nge7 offers White several setups: 7. Bc4 is the main line, and the alternatives are 7. g3, the Meitner Variation 7. Nc2, the Paulsen Attack 7. Bb5 and the Blackburne Attack 7. Qd2. Moving the queen on with 6...Qg6 is called the Millennium Variation. The Blumenfeld Attack, 5...Qf6 6. Nb5, is considered inferior. After 6...Bxe3 7. fxe3 Qh4+ 8. g3 Qxe4, the attempt 9. Nxc7+ Kd8 10. Nxa8 traps White's knight, and 10...Qxh1 takes a rook in return. 5. Be3 is the engine's first choice and scores better for White than the exchange on c6.

The Potter Variation, 5. Nb3, keeps the pieces on and gains a tempo on the bishop. Black usually answers 5...Bb6, while 5...Bb4 is the Romanishin Variation. Other ways of rescuing the knight give Black more time. 5. Nf5 hopes for 5...Nf6? 6. Nxg7+ Kf8 7. Bh6 with the threat of Ne6+, but 5...d5 refutes it. 5. c3 develops nothing and takes the c3-square from White's knight, which leaves e4 harder to defend. Black can then aim at the pawn with 5...Nf6 or 5...Qh4. One line runs 5...Qh4 6. Bd3 Nxd4 7. cxd4 Bxd4 8. O-O, and Black is a pawn ahead.