The foundation of QuickZip® Crib Sheet, the zipper base fits snuggly around the mattress and allows the quick on-and-off of the zipper sheet. These machine-washable, 220 thread-count bases will not shrink even after multiple washings. Having extra zipper bases is handy for anyone who travels, drops off sheets for daycare or at the babysitter, or just to have the peace of mind that you’ll always have QuickZip® convenience even when your other zipper base is in the laundry.