In the case of Mongolian nomads, a family moves twice a year. These two movements generally occur during the summer and winter. The winter destination is usually located near the mountains in a valley and most families already have fixed winter locations. Their winter locations have shelter for animals and are not used by other families while they are out. In the summer they move to a more open area that the animals can graze. Most nomads usually move in the same region and don't travel very far to a totally different region. Since they usually circle around a large area, communities form and families generally know where the other ones are. Often, families do not have the resources to move from one province to another unless they are moving out of the area permanently. A family can move on its own or with others; if it moves alone, they are usually no more than a couple of kilometers from each other. The geographical closeness of families is usually for mutual support. Pastoral nomad societies usually do not have a large population. One such society, the Mongols, gave rise to the largest land empire in history. The Mongols originally consisted of loosely organized nomadic tribes in Mongolia, Manchuria, and Siberia. In the late 12th century, Genghis Khan united them and other nomadic tribes to found the Mongol Empire, which eventually stretched the length of Asia.