Wikipedia
Ü-Tsang or Tsang-Ü, is one of the three traditional provinces of Tibet, the other two being Amdo and Kham. Geographically Ü-Tsang covered the central and western portions of the Tibetan cultural area, including the Brahmaputra River watershed, the western districts surrounding and extending past Mount Kailash, and much of the vast Changtang plateau to the north. The Himalayas defined Ü-Tsang's southern border. The present Tibet Autonomous Region corresponds approximately to what was ancient Ü-Tsang and western Kham.
Ü-Tsang was formed by the merging of two earlier power centers: Ü in central Tibet, controlled by the Gelug lineage of Tibetan Buddhism under the early Dalai Lamas, and Tsang which extended from Gyantse to points west, controlled by the rival Sakya lineage. Military victories by the powerful Khoshut Mongol Güshi Khan that backed 5th Dalai Lama and founded Ganden Phodrang government in 1642, consolidated power over the combined region, followed by the rule of the Qing Dynasty started in 1720 by the Qianlong Emperor that continued until the British expedition to Tibet (1903-1904).
Ü-Tsang is the cultural heartland of the Tibetan people. Successive Dalai Lamas have ruled Tibet from the Potala and Norbulingka palaces in Lhasa. Jokhang, perhaps the most holy temple in Tibetan Buddhism, is also located there. The Lhasa dialect is used as a lingua franca in Ü-Tsang and the Tibetan Exile koiné language is also based largely on it.