Stacking bookcase units used in early twentieth-century houses included individual units for books that could be stacked one on another depending on the size of the library to be accommodated. Such systems often included optional single glass-fronted doors that were hinged on the sides near the top of each case. These wide glass doors gave book owners unbroken visual access to their libraries while protecting the contents from damage from dust and dampness. Each individual "self-retiring" door could swing out and up to a horizontal position and slide back over the books on the shelf, thereby being contained within the cabinet when not in use. This hinged self-storing system offered additional protection for the glass. It also allowed the cabinets to occupy less space since large doors never swung out into the room to obstruct the use of the cabinet.