Classical Variation, Main Line

With 6. h4, White threatens h5, which would trap the bishop on g6. It also claims kingside space before Black has committed to a plan. Black's usual answer is 6...h6, chosen in roughly four games out of five. It gives the bishop the h7 retreat, and after 7. Nf3 Nd7 8. h5 Bh7 play enters the Spassky Variation and the familiar main-line structures. White's pawn on h5 then cramps Black's kingside, but it can become a target in an endgame.

The second most common reply is 6...h5. It stops the pawn from advancing at once, but it gives up control of g5 permanently, and White scores noticeably better against it in the Lichess database. A less common choice is 6...Nf6. It develops first and allows h5, so the bishop has to find another square. Engines rank 6...h6, 6...h5 and 6...Nf6 as the top three moves, in that order. They give White a slight edge in the starting position, and practical results also lean somewhat toward White.