Complex sentence: Rectangular standard bed pillow cases usually do not have zippers, but instead have one side open all the time, however, a zippered pillow protector is often placed around standard pillows with the case in turn covering the protector.
Split into simpler sentences: 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.