Pillows consist of a filler material enclosed in a fabric cover or shell. Covers are made of cloth, such as silk, known as the pillow case or pillow slip. Some pillows have a fancier cover called a sham which is closed on all sides and usually has a slit in the back through which the pillow is placed. Rectangular standard bed pillow cases usually do not have zippers, but instead, have one side open all the time. Often, a zippered pillow protector is often placed around standard pillows with the case in turn covering the protector.