Iran is located in southwest Asia and borders the Gulf of Oman, Persian Gulf, and Caspian Sea. Its mountains have helped to shape both the political and the economic history of the country for several centuries. The mountains enclose several broad basins, or plateaus, on which major agricultural and urban settlements are located. Until the 20th century, when major highways and railroads were constructed through the mountains to connect the population centers, these basins tended to be relatively isolated from one another.
Typically, one major town dominated each basin, and there were complex economic relationships between the town and the hundreds of villages that surrounded it. In the higher elevations of the mountains rimming the basins, tribally organized groups practiced transhumance, moving with their herds of sheep and goats between traditionally established summer and winter pastures. There are no major river systems in the country, and historically transportation was by means of caravans that followed routes traversing gaps and passes in the mountains. The mountains also impeded easy access to the Persian Gulf and the Caspian Sea.
With an area of 1,648,000 square kilometres (636,000 sq mi), Iran ranks eighteenth in size among the countries of the world. Iran is about one-fifth the size of the continental United States, or slightly larger than the combined area of the western United States (Arizona, California, Oregon, Washington, Nevada, and Idaho).
Iran shares its northern borders with three post-Soviet states: Armenia, Azerbaijan, and Turkmenistan. These borders extend for more than 2,000 kilometres (1,200 mi), including nearly 650 kilometres (400 mi) of water along the southern shore of the Caspian Sea. Iran’s western borders are with Turkey in the north and Iraq in the south, terminating at the Shatt al-Arab, which Iranians call the Arvand Rud.
The Persian Gulf and Gulf of Oman littorals form the entire 1,770 kilometres (1,100 mi) southern border. To the east lie Afghanistan on the north and Pakistan on the south. Iran’s diagonal distance from Azerbaijan in the northwest to Sistan and Baluchestan Province in the southeast is approximately 2,333 kilometres (1,450 mi).