A sliding door is a type of door which opens horizontally by sliding, usually parallel to a wall. Sliding doors can be mounted either on top of a track below or be suspended from a track above. Some types slide into a space in the parallel wall in the direction of travel, rather than the door sliding along the outside of the parallel wall. There are several types of sliding doors, such as pocket doors, sliding glass doors, center-opening doors, and bypass doors. Sliding doors are commonly used as shower doors, glass doors, screen doors, and wardrobe doors, and in vans.