Shelves are usually one of three types: rail, solid or wire. Shelves can be metal or plastic, and can either slide on to posts to form vertical shelving units, or attach directly to a wall for over-hang shelves where an entire unit could not fit. In addition, there are parts that can be attached to dunnage racks to increase shelving space low to the ground. All of these options are available for you to choose from, depending on how you have your storage area arranged.