The land of rTsaṅ/sTsaṅ frequently mentioned in the Tun-huang documents is not the same as the present-day gTsaṅ in Western Tibet, but a region partly overlapping Yan lag gsum paḥi ru/Sum ru, one of the wings (ru) organized after the death of King Sroṅ brstan sgam po. The account of Sum ru’s boundaries in lHo brag chos ḥbyun enables us to determine its eastern end to be in the upper reaches of the Chin-sha River. Also location of mThoṅ, Khyab and rGya, indicated in other sources as tribes bounding on Sum ru, leads us to the same conclusion. Sun-po 孫波, which is said in Chinese sources to be a new name for the Su-p’i 蘇毗, seems to transcribe gSum pa occurring in the name Yan lag gsum paḥi ru. Further it is possible that Su-p’i comes from Seḥu Phyvaḥ/So bya/So byi, an amalgamation of the tribes Seḥu of rGod and Phyvaḥ of rTsaṅ. This rGod, which later becomes one of the main tribes forming His-hsia 西夏 (Mi ñag), appears to have moved eastward from Central Tibet into that region around the time when the Ancient Tibetan Kingdom was founded.The appellation rTsaṅ chen suggests to us existence of a region generally called rTsaṅ extending to the north of Yan lag gsum paḥi ru. The headwaters of the Huang-ho, referred to as the place where Princess Wen-ch’eng arrived at in her journey to meet the royal husband, was adjacent to this rTsaṅ yul.
