Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. d4/3...exd4/4. Nxd4/4...Bc5

In the classical continuation of the Scotch, 4...Bc5 attacks White's central knight and puts indirect pressure on the f2 square.

White has three continuations of roughly equal popularity.

5. Be3 defends the knight and relieves the pressure on f2. It threatens a tactic to win a piece: if, say, 5...Nf6?? 6. Nxc6! (trades with tempo on Black's queen) dxc6 7. Qxd8+ Kxd8 8. Bxc5 wins a bishop. The main move is 5...Qf6, adding pressure to f2 and another attacker to White's knight, which White defends with 6. c3.

6. Nb5?!, the Blumenfeld attack, which threatens Nxc7+ to win the rook, is inferior due to the continuation 6...Bxe3 (threatening 7...Qxf2#) 7. fxe3 Qh4+ 8. g3 Qxe4, and if 9. Nxc7+ Kd8 10. Nxa8 White's knight is trapped and Black can pick up a rook themselves with 10...Qxh1.

5. Nxc6 trades off the knight immediately with tempo on the queen. As 5...dxc6?! allows a queen trade where Black loses castling rights, the main move is 5...Qf6, called the intermezzo variation, which threatens 6...Qxf2#.

Finally, 5. Nb3, the Potter variation, saves the knight while winning a tempo on Black's bishop.

Other ways to save the knight are less accurate and allow Black to build on their lead in development.

• 5. Nf5 sets a trap: 5...Nf6? 6. Nxg7+ Kf8 7. Bh6, threatening 8. Ne7+ winning the queen. However, it is losing to 5...d5, intending 6...Bxf5 and extending Black's development lead.
• 5. c3 defends the knight but doesn't develop a piece. Though this looks like it would transpose to the main line after 5...Qf6 6. Be3, Black has other options including attacking the e4 pawn and taking advantage of the fact that White can no longer play Nc3 to defend it: 5...Qh4 or 5...Nf6. 5...Qh4 6. Bd3 Nxd4 7. cxd4 Bxd4 8. O-O, Black is up a pawn.

1. e4 e5 2. Nf3 Nc6 3. d4 exd4 4. Nxd4 Bc5

Wikipedia has related information at Scotch Game, Classical Variation.