Abstract: In a disk type sheet inverting and stacking system with rotatable disk units with sheet transporting slots, in which printed sheets outputted by a reproduction system are sequentially fed into those slots to be inverted by rotation of the disk units and then released from the slots at a stacking position, the sheet transporting slots of the disk units are provided with a convolute sheet path formed by plural internal sheet corrugating protuberances extending transversely across the slot from opposite sides of the slot, spaced along the slot, to form a convolute sheet corrugating sheet path within the slot to automatically increase the retention force of the sheet within the slot in proportion to an increase in the thickness or stiffness of the sheet, without substantially increasing the resistance to lateral movement of the sheet within the slot.