Tipis consist of four elements: a set of ten to fifteen sapling poles, a canvas or skin cover (the outer shape familiar from photographs), an inner canvas or skin lining, and a canvas or skin door. Ropes and pegs are required to bind the poles, close the cover, attach the lining and door, and anchor the resulting structure to the ground. Tipis are distinguished from other tents by two crucial innovations: the opening at the top and the smoke flaps, which allow the dweller to cook and heat themselves with an open fire, and the lining, which supplies a steady, controlled flow of fresh air to fire and dwellers in almost any weather. Tipis are designed to be easily set up to allow camps to be moved to follow game migrations, especially the bison. The long poles could be used to construct a dog or later horse-pulled travois.
Tipi covers are made by sewing together strips of canvas or hide and cutting out a semicircular shape from the resulting surface. Trimming this shape yields a door and the smoke flaps that allow the dwellers to control their fires. The lining is the most difficult element to measure, since it consists of lozenge-shaped strips of canvas assembled to form the shape of a truncated cone. The poles, made of peeled, polished and dried saplings, are cut to measure about six feet more than the radius of the cover. From Wikipedia.com