Gunsberg Variation

The Gunsberg Variation, 4. a3, spends a tempo on a waiting move that stops ...Bb4. Since the position is symmetrical, Black effectively becomes the first player in a Four Knights Game, and the extra a3 matters mainly because it removes the pin. It has been a specialty of the Polish grandmaster Paweł Blehm. Engines rate the position as balanced and favor 4...d5.

The most frequent reply is 4...Bc5, but it walks into the fork trick with colors reversed: 5. Nxe5 Nxe5 6. d4 Bd6 7. f4 Nc6 8. e5 gains central space with tempo. The principled answer is 4...d5, producing a reversed Scotch Four Knights. A sample line runs 5. exd5 Nxd5 6. Bb5 Nxc3 7. bxc3 Bd6 8. d4 exd4 9. cxd4 O-O 10. O-O, with an equal game. Black scores better after 4...d5 than after the other main replies. The modest 4...d6 is also played, and 4...d6 5. h3, with White adding another waiting move, is known as the Provincial Opening.