Mobile homes or static caravans are prefabricated homes built in factories, rather than on site, and then taken to the place where they will be occupied. They are usually transported by tractor-trailers over public roads to sites which are often in rural areas or high-density developments. In some countries they are used for temporary accommodation on campsites. While these houses are usually placed in one location and left there permanently, they do retain the ability to be moved as this is a requirement in many areas. Behind the cosmetic work fitted at installation to hide the base, there are strong trailer frames, axles, wheels and tow-hitches. The two major sizes are single-wides and double-wides. Single-wides are eighteen feet or less in width and 90 feet or less in length and can be towed to their site as a single unit. Double-wides are twenty feet or more wide and are 90 feet in length or less and are towed to their site in two separate units, which are then joined together.