An early type of caravan is the horse-drawn covered wagon, which from circa 1745 played a significant part in opening up of the interior of the North American continent to white settlement. By the 1920s the RV was well established in the United States, with RV camping clubs established across the country, despite the unpaved roads and limited camping facilities. Several companies began manufacturing house trailers (called trailer coaches at the time). Airstream is one such company. Until the 1950s, the RV industry was closely connected to the mobile home industry because most mobile homes were shorter than 9 metres (30 ft) long, and thus easily transportable. During the 1950s, the RV and mobile home industries became separated and RV manufacturers began building self-contained motorhomes.