From a distance the tents looked like great black spiders. Their coverings were made from the long hair of the yak, loosely woven into a coarse cloth. Roofs were flat or slightly sloping from the centre opening - about two feet wide and about six feet long-to allow the smoke to escape and the light to come in (p. 151).
Source of Descriptive Information
Forman, H. (1936). Through Forbidden Tibet. London: Jarrolds