The flexible 5.0-0 is sometimes called the Barendregt Variation, but it was Fischer who developed it into a serious weapon in the 1960s. Unlike 5.d4, it forces Black to defend the e-pawn, usually with 5...f6, 5...Bg4, 5...Qd6 (the sharpest line, preparing queenside castling), 5...Qe7, 5...Qf6 or 5...Bd6. A rare but playable move is 5...Be6 (or 5...Be7), the idea being that if White plays 6.Nxe5, Black plays 6... Qd4, forking the knight and the e4-pawn. The move ...Qd4, regaining the pawn at e4, is usually impossible in these variations once White has castled, due to the open e-file.