The Polarshield 4' x 4' Air Pillow for Above Ground Pool Winter Covers is made of rugged 16-gauge vinyl. The Winter Cover Air Pillow is designed to displace the pressure of ice freezing to help protect your pool walls. The Air Pillow also reduces excess rain, snow, and leaves on top your winter cover.What size air pillow should I use with my above ground pool winter cover?4' x 4' Air Pillow works great with 24' or smaller Round Above Ground Pools4' x 8' Air Pillow is recommended for 28' Round and small Oval Above Ground Pools4' x 15' Air Pillow is for use with 15' x 30' or larger Above Ground Pools