Container vessels eliminate the individual hatches, holds and dividers of the traditional general cargo vessels. The hull of a typical container ship is a huge warehouse divided into cells by vertical guide rails. These cells are designed to hold cargo in pre-packed units – containers. Shipping containers are usually made of steel, but other materials like aluminum, fiberglass or plywood are also used. They are designed to be entirely transferred to and from smaller coastal carriers, trains, trucks or semi-trailers (and so are carried by different "modes" of transport during one voyage, thus giving the name intermodal transport). There are several types of containers and they are categorized according to their size and functions.