If each side has a rook in addition to the bishop, the stronger side has many more winning prospects. The attacking rook can have influence on both color of squares. Sometimes exchanging the rook for the defender's bishop breaks a fortress. Sometimes the defending bishop can be sacrificed for pawns to result in a rook and bishop versus rook endgame that can be drawn (see pawnless chess endgame). The most difficult problem encountered by the stronger side is usually in breaking a blockade by the opposite bishop. These endgames occur in 2.8% of the games between high-rated players .