Rubinstein Variation, Fort Knox Variation

The Fort Knox Variation arises after 3...dxe4 4. Nxe4 Bd7 5. Nf3 Bc6. Black spends two tempi to bring the light-squared bishop to the long diagonal before developing the kingside. On c6 the bishop no longer sits behind the e6-pawn. It bears down on the centralized knight and offers to trade itself for it, which removes the piece that most often stays passive in the French. The cost is time. White keeps a space advantage and the more harmonious development, while Black accepts a compact position with few targets. Alexander Rustemov is the line's best-known exponent.

White nearly always answers 6. Bd3, which protects the knight with a developing move. After this, ...Bxe4 lets White recapture with the bishop and keep a strong piece on e4. The retreats 6. Ng3 and 6. Nc3 avoid the exchange altogether, but they are much less common. The engine gives White a slight edge, yet in Lichess games Black scores marginally better than White. The variation is a low-theory choice that suits players who want a solid French structure without much risk.