"An Historical Atlas of Central Asia" written by Yuri Bregel. This is stated on Page no 39 of this book.

The empire created by Chingis Khan was considered, according to the Inner Asian nomadic tradition, the common patrimony of the ruling house, and after the death of Chingis Khan in Mongolia in 1227 it was divided among the members of his clan, mainly among the four sons of his senior wife. Their appanages were known by the term ulus, which referred to their nomadic subjects, but later this term was associated with the territory as well. An appanage was limited, at least initially, to the pasture lands; the sedentary regions were under the joint authority of the ruling clan.
The initial distribution of the main uluses was made by Chingis Khan himself. The eldest son, Jochi, was assigned, according to custom, the part of his father’s possessions that was the farthest from Mongolia—the Irtïsh valley, together with the lands yet to be conquered “as far as the hoofs of the Mongol horses had reached,” that is, the westernmost regions of the Mongol Empire. Jochi died before his father, and Chingis Khan stipulated that
his ulus should be inherited by Jochi’s second son, Batu. Chingis Khan’s second son, Chaghatay, initially received the lands north of Mavarannahr, more or less coinciding with the former Qara-Khitay country, to which Mavarannahr was soon added.
The third son, Ögedey, received Jungharia; the youngest son, Toluy, inherited the heartland of the empire, Mongolia. The sedentary regions had a separate administration (which was responsible mainly for tax collection and other financial matters); in Central Asia it was headed by a local Muslim merchant, Mahmud Yalavach Khorezmi, and after his transfer to China for a similar job, by his son Mas#ud Bek. Ögedey was designated by Chingis Khan to be his successor; he was confirmed in this post by the assembly (quriltay) of Mongol princes in 1229 and adopted the title of qa"an, or “great khan.” Under him the Mongol expansion continued: northern China was finally conquered in 1234, the Qïpchaq steppe and the Russian principalities were conquered in 1237-1241, and most of Iran by 1241. After a long interregnum following the death of Ögedey in 1241 (and the death of Chaghatay in 1242) and the short and ineffectual rule of Ögedey’s son Güyük (1246-1248), Batu nominated for the post of great khan the eldest son of Toluy, Möngke, who was elected at a quriltay held in Batu’s territory and then reconfirmed at another one in Mongolia in 1251. The descendants of Ögedey and Chaghatay, most of whom opposed this election, were executed or exiled; the ulus of Ögedey was effectively dissolved, and much of its territory incorporated into the domain of the new qa"an, Möngke. The ulus of Chaghatay remained (because its senior prince supported Möngke), but it was significantly weakened, and the Jochids extended their control to Western Turkestan, so that for some contemporary European observers the Mongol Empire at that time appeared to be a condominium of Batu and Möngke (which it was not). Under Möngke the empire continued to expand until his death (in 1259): the rest of China was conquered by Monge's brother Quigley, founding a new ulus and soon adopting the Chinese dynastic name Yuan, while another brother, Hülegü, conquered Western Iran and Iraq and destroyed the Abbasid caliphate in 1258, founding also a new ulus which became known as that of the Ilkhans
(from the title used by its rulers)