Mongols have been living in virtually the same dwellings since at least the 6th century AD. These dwellings are called gers, and during the Mongol Empire they consisted of a round, collapsible wooden frame covered in felt. The roof was formed from about 80 wooden rods attached at one end to the wall frame and at the other to an iron ring in the center, providing a sturdy base for the felt roof. Without the roof in place, this frame would have resembled a large wooden wheel with the wooden spokes converging at the iron ring. The top of the roof was usually about five feet higher than the walls so precipitation would run to the ground. The ring at the peak of the yurt could be left open as a vent for smoke and a window for sunlight, or it could be closed with a piece of felt. Doors were made from a felt flap or, for richer families, out of wood.