Morphy Defense, Mackenzie Variation

The Mackenzie Variation, 5. d4, also called the Center Attack, strikes at e5 at once instead of castling, aiming to use the e-file before Black's king is safe. Black usually takes with 5...exd4, and 6. O-O Be7 reaches a position also arising from the Closed Defense via 5. O-O Be7 6. d4 exd4. The alternative 5...Nxe4 6. O-O transposes to the Open Defense. Named after George Henry Mackenzie, it leads to sharp play but, by modern theory, gives White no advantage.