The DaKine Aero Rack Pads are use to pad the top of an existing roof rack on a car or SUV. They come in a pack of two, are 18" wide, and the Regular size can fit most roof racks. For larger roof racks, like on a Hummer, we recommend the XL. The pads are the same size on both the Regular and XL but the Velcro sleeve is wider on the XL to accommodate thicker bars. Both are made from custom extruded foam and durable 600D polyester fabric.