Tarrasch Variation, Open System, Euwe-Keres Line

Instead of resolving the center with 4. exd5, White first develops the king's knight so that it guards d4 and can recapture there. The tension stays in place, so Black has to commit first. Much of the play transposes to the 4. exd5 systems, but White can sidestep some of them. The engine rates the position as roughly balanced, and online results are close to even.

In online play 4...Nc6 is the most common reply, followed by 4...cxd4. After 4...Nc6 5. exd5, Black normally recaptures with the pawn and reaches the Main Line. After 4...cxd4, White can enter the queen-recapture lines with 5. exd5 Qxd5 or avoid them with 5. Nxd4, which sets up Nb5 aiming at c7 and d6. One illustrative line is 5...dxe4 6. Nxe4 Nf6 7. Nxf6+ Qxf6 8. Nb5 Na6 9. Nd6+ Bxd6 10. Qxd6, where White keeps the bishop pair and the better game. Against 4...Nf6, White chooses between 5. e5, a Closed Tarrasch structure that includes the Korchnoi Gambit, and the open 5. exd5. The rare 4...a6 is usually answered by 5. exd5 or 5. dxc5.