Ulus is a key concept for understanding the Mongol Empire; the Mongols called the empire they had created the ‘Mongol Ulus’ or the ‘Yeke (= Great) Mongol Ulus.’ It was a single huge ulus containing within itself a large number of multilayered smaller uluses. The sizes of these uluses differed. The three large western uluses of Chaghadai, Jochi, and Hülegü were so large that they looked like independent empires, and in fact they were politically quite autonomous from the Qa’an Ulus in the east. However, their enormous size or political independence notwithstanding, to the Mongols of that time they were after all individual uluses comprising part of the Yeke Mongol Ulus. The notion of ulus appears to have been an important basis that enabled the Mongols to retain a sense of unity within their empire.