A device for opening folders (24) during transportation comprises an opener (91) arranged to run together along and next to the transversally central area of the folder tail. The opener is provided with a wedge (132) that is driven into the folder tail. The wedge has two support edges (137) that diverge from the wedge tip (138) in a direction opposite the drive-in direction of the wedge in the folder, away from each other in the plane of the folder, as well as away from the extension plane of the folder so that an outer folder part (24A) deflected upon the driving-in of the wedge (132) will be supported by the wedge support edges (137) at separated positions along the folder tail, whereby a subjacent inner folder part (24B) is exposed so that a gripper (23) on a leading wall (21) in a folder conveyor can fix the subjacent inner folder part (24B) against the leading wall (21).