The simplest throw pillows, however, do not have an opening and are often loose filled, meaning there is no insert. These cushions do not feature any additional decoration such as tassels or piping. These designs are mostly aimed at beginners to sewing. Typically, however, throw pillows combine a removable cover and an insert to make up the finished product. The cover can easily be removed and washed, or the insert changed if it becomes too flat or triggers allergies. The covers, commonly referred to as throw pillow covers or cushion covers, are often sold separately from the insert. These covers have a zipper opening, which comes in two variations: an envelope, which is used to conceal the zipper behind, and a hidden opening, which is found at the bottom, reverse of the cushion.