The Swimline 21 Foot Round Above Ground Winter Swimming Pool Cover is great for keeping your pool in good condition during the cold, winter months, but it will also make getting your pool back into shape in the spring much easier. This pool cover is made of laminated polyethylene sheeting woven with thick, high density polyethylene stitching, which ensures superior tensile strength and durability. It has also been treated to provide maximum resistance to damaging ultraviolet rays. For the long life of your pool, choose the Swimline 21 Foot Round Above Ground Winter Swimming Pool Cover.