French Defence, Winawer Variation
This is the main line of the Winawer Variation. White pushes his e pawn forward, gaining space and defending it with the d4 pawn. The pawn structure now is the same as the Advance Variation, with white gaining extra space on the kingside. Black's usual response is 4...c5, threatening white's pawn chain at its base. Note that since white's knight is already on c3, the standard Advance Variation reply of c3 is unavailable. Thus, since c5 also cuts off the bishop's escape route, white usually chooses to boot the bishop with 5.a3, leading to an exchange of pieces with 5...Bxc3+ 6.bxc3.