Pillow cases are covers that are designed to be slipped over pillows. There are a number of reasons to use them, ranging from the desire to maintain a hygienic environment to a cheap way to switch up interior decor. Many home supply stores sell pillow cases, and it is also possible to make one by hand, for people who want to use specific fabrics or create special designs. There are a wide range of styles and types available.
One of the main reasons to use pillow cases rather than using pillows as-is is hygiene. A case can easily be removed for washing or replacement, while pillows are challenging to wash. They trap grease from hair along with dirt and other materials, preventing them from reaching the pillow filling. Using pillow cases also reduces the risk of spreading certain diseases, as they can be changed between users so that people don't pick up lice, scabies, colds, and a wide variety of other unpleasant medical problems from used pillows.
People with allergies may use specialized pillow cases that are designed to reduce allergies. These can include hypoallergenic pillow covers that prevent dust mites from getting out of the pillow filling, along with those made from hypoallergenic materials which will not trigger an allergic event.
Aesthetic reasons can be another factor in the use of pillow cases. A typical pillow is not very exciting to look at, since it's simply a stuffed packet of plain material. A cover can totally change the look and feel of a pillow, and people can change the cases on ornamental pillows to create a change of mood in a public room. For example, people might use colorful silk ones on throw pillows in the living room to liven the room up for guests, or change the covers on bed pillows to match a change of bedding.
Some pillow cases zip up, snugly covering the pillow inside. Those with hook-and-loop fasteners, ties, and other fastenings are available. Others may be open, or designed with a small flap which folds over the pillow once it has been inserted into the cover. An advantage to those that close is that the pillow will not be exposed during use, which can be a problem with open ones. However, open pillow cases fit a range of pillow types and sizes, since they are not designed for a snug fit, so they can be more versatile.