Semu is the name of a caste established by the Yuan dynasty. The 31 Semu categories referred to people who came from Central and West Asia. They had come to serve the Yuan dynasty by enfranchising under the dominant Mongol caste. The Semu were not a self-defined and homogeneous ethnic group per se, but one of the four castes of the Yuan dynasty: the Mongols, Semu, the "Han" and the Southerners. Among the Semu were Buddhist Turpan Uyghurs, Tanguts and Tibetans; Nestorian Christian tribes like the Ongud; Alans; Muslim Central Asian Turkic peoples including the Khwarazmians and Karakhanids; West Asian Arab, Jewish, Christians and other minor groups who are from even farther west.

The Tangut people were a Sino-Tibetan people who founded and inhabited the Western Xia dynasty. The group initially lived under Tuyuhun authority, but later submitted to the Tang dynasty. After the collapse of Tang dynasty, the Tanguts established the Western Xia. They spoke the Tangut language, which was previously believed to be one of the Qiangic languages or Yi languages which belong to the Tibeto-Burman family." Phylogenetic and historical linguistic accounts, however, reveal that Tangut belonged instead to the Gyalrongic branch of Tibeto-Burman. Western Xia was annihilated by the Mongol Empire in 1227, most of its written records and architecture were destroyed. Today the Tangut language and its unique script are extinct; only fragments of Tangut literature remain.