These require either an aluminium or timber mid rail to fit the system to the structure. Base rails are not essential, but may be beneficial on certain surfaces as well as improving appearance.The system works by the film being wound around a tube, which is turned by a small gearbox unit. The unit is manually wound from within the structure. Permanent netting is fixed to the outside of the vent. It is a very clean neat system and works well on curved and vertical sides.