Fianchetto Variation, Nimzowitsch Variation

The Nimzowitsch Variation (ECO E15) is Black's most direct answer to the fianchetto. Instead of contesting the long diagonal with 4...Bb7, the bishop goes to a6 and hits c4 immediately. Because White's king's bishop is heading for g2, the natural defense e3 fits the setup poorly, so the pawn has to be covered by other means. Wikibooks treats the move as the Modern Main Line, the successor to the older 4...Bb7. The engine rates the position as roughly balanced.

The most common reply is 5. b3, the Quiet Line, which protects c4 with a pawn at the cost of loosening the c3 square and the long diagonal; 5...Bb4+ then gives the Check Variation. The other main approach uses the queen. 5. Qc2 guards the pawn along the second rank, while 5. Qa4, the Nimzowitsch Attack, defends c4 and attacks the bishop on a6 at the same time. 5. Qb3, Timman's Line, is rarely seen but scores well for White. The engine also favors 5. Nbd2, which covers c4 with the knight. By contrast, 5. Bg2 leaves the pawn undefended, and Black scores well after it in the database.