A shelf can be attached to a wall or other vertical surface, be suspended from a ceiling, be a part of a free-standing frame unit, or it can be part of a piece of furniture such as a cabinet, bookcase, entertainment center, some headboards, and so on. Usually two to six shelves make up a unit, each shelf being attached perpendicularly to the vertical or diagonal supports and positioned parallel one above the other. Free-standing shelves can be accessible from either one or both longer length sides. A shelf with a hidden internal bracket is termed a floating shelf. A shelf or case designed to display books is a bookshelf. The length of the shelf is based upon the space limitations of its siting and the amount of weight which it will be expected to hold. The vertical distance between the shelves is based upon the space limitations of the unit's siting and the height of the objects; adjustable shelving systems allow the vertical distance to be altered. The unit can be fixed or be some form of mobile shelving. The most heavy duty shelving is pallet racking. In a store, the front edge of the shelf under the object(s) held might be used to display the name, product number, pricing, and other information about the object(s).