Morphy Defense, Anderssen Variation

The Anderssen Variation, 5. d3, is the most frequent alternative to 5. O-O. By protecting e4, White turns Bxc6 followed by Nxe5 into a genuine threat, because the reply ...Qd4 no longer hits an undefended e-pawn. Black must therefore cover e5 or drive away the bishop. The move rules out the Open Defense. A drawback is that after 5...Bc5 it is unclear whether White keeps any advantage, so players seeking a d3 setup often prefer 5. O-O and 6. d3 after 5...Be7, the Martinez Variation. Engines rate the position as balanced.

The usual reply is 5...b5, which removes the threat but gives White a target for a later a4. After 6. Bb3, Black normally plays 6...Bc5, while 6...Be7 7. O-O transposes to the Martinez. With 5...d6, Black intends 6...b5 7. Bb3 Na5. White can answer 6. c3, reaching the Martinez after 6...Be7, or play 6. c4, the Duras Variation, which prevents ...b5. On 5...Bc5, the sequence 6. Bxc6 dxc6 7. Nxe5 runs into 7...Qd4, hitting the knight and f2. A typical line is 6. c3 b5 7. Bb3 d6 8. O-O O-O 9. Bg5 h6 10. Bh4 g5, after which White chooses between 11. Bg3 and the sharper 11. Nxg5.