Down pillows are softer than down alternative pillows yet do not provide as much support. However, a down pillow with higher fill power can be used for more firmness. Feather pillows compare to down pillows in softness yet provide more support and often come with quilted layers to protect against the feathers. The pillow cover is the shell of the pillow, rather than the pillowcase, which goes over the pillow cover. Pillow covers with a higher thread count are both softer and seal the filling of the pillow better. Cotton and polyester pillow covers are most common, and some pillow covers may be waterproof. Pillow covers for foam and memory foam pillows are often removable. Down alternative pillows can be filled with polyester fiberfill or gel fiber, which can come in different densities to give medium to high firmness, and can also offer unique properties such as squishy feel. Down alternative bedding materials such as PrimaLoft can provide down-like insulation and loft to pillows. Unique fillings such as a coil of foam around a polyester center can be found. A D-core pillow has an indentation and is like a neck pillow in that it supports the neck and alleviates snoring. A water pillow has an insert inside its filling which is filled with water. A water pillow can be used with hot or cold water depending on the season. Magnetic body pads use magnets to reduce swelling, ease joints, improve circulation, and relieve fatigue. A floor pillow provides a good alternative to a bean bag due to its superior comfort. Floor pillows are thick and square, can be three feet wide, and are great for relaxing. A bed rest is a vertical pillow with side extensions for sitting up in bed.