King's English Variation, Four Knights Variation, Flexible Line

The Flexible Line is the third most common fourth move in the English Four Knights Variation, after 4. g3 and 4. e3. With 4. d3 White takes control of e4. This blunts the plan of ...Bb4 followed by ...e4, one of Black's main ideas in the parent position. It also opens the c1-bishop's diagonal and leaves White free to choose between a kingside fianchetto, a later e4, or a different setup depending on Black's reply. The move commits to little, but it also does nothing to stop Black's freeing break in the center.

Black's most popular answer is still 4...Bb4, pinning the c3-knight and keeping ...Bxc3 in hand to double White's pawns. 4...Bc5 places the bishop on the active a7-g1 diagonal, while 4...d5 strikes at the center at once. The engine prefers 4...d5, and it is the only one of the three main replies after which Black scores slightly better than White. The position is considered roughly balanced. In practice White scores a little better overall, and draws are rare.