Shahsevan nomads commonly join forces in cooperative herding and camping groups of three to five households. Such a herding unit usually camps on its own in the mountain pastures between June and early September but joins with one or two others to form a winter camp of ten to fifteen households during the period from November to April. Two to three such winter camps comprise the basic nomadic community ( tireh ), which moves and camps as a unit during the autumn migration in October and the spring migration in May. Apart from their distinctive frontier location and history, the most obvious distinguishing characteristic of the Shahsevan with respect to other nomads is the type of tent they erect. Most nomads in Iran live in rectangular goat-hair tents, similar to those of Arab Bedouin. The Shahsevan tent ( alaçigh ) resembles an upturned saucer and is related to the Central Asian yurt. A wooden framework of curved struts, held together by long girths, radiates out and down to the ground from a central roof ring, which is itself anchored to the ground by a massive peg. A covering of thick felt mats keeps out both heat and cold. The hearth, focus of all domestic life, lies between the doorway and the central peg. The wooden parts, which are bought in the bazaar, are expected to last two years, and the felts, which need replacing every three years, are made in camp from the wool of the flocks. Such a tent or hut, heavier and sturdier than other types, is very expensive to maintain, and only two out of three families can afford one: poorer families use a much smaller, simpler, and cheaper construction ( kümeh ) .