Yabgu (Old Turkic: , yabγu, Traditional Chinese: 葉護, Simplified Chinese: 叶护, Jabgu, Djabgu, literally, "pioneer", "guide") was a state office in the early Turkic states, roughly equivalent to viceroy. The title carried autonomy in different degrees, and its links with the central authority of Khagan varied from economical and political subordination to superficial political deference.
The position of Yabgu was traditionally given to the second highest member of a ruling clan (Ashina), with the first member being the Kagan himself. Frequently, Yabgu was a younger brother of the ruling Kagan, or a representative of the next generation, called Shad (blood prince). Mahmud Kashgari defined the title Yabgu as "position two steps below Kagan", listing heir apparent Shad a step above Yabgu.
As the Khaganate decentralized, the Yabgu gained more autonomous power within the suzerainty, and historical records name a number of independent states with "Yabgu" being the title of the supreme ruler. One prominent example was the Oguz Yabgu state in Middle Asia, which was formed after the fragmentation of the Second Türkic Kaganate in the 840es. Another prominent example was the Karluk Yabgu, the head of the Karluk confederation which in the 766 occupied Suyab in the Jeti-su area, and eventually grew into a powerful Karakhanid state.
Although believed by some to be a derivation from early Turkic davgu, most scholars believe that that the word Yabgu is of Indo-European origin, and was perhaps borrowed by the Türks from the Kushan political tradition, preserved by the Hephtalites.
Friedrich Hirth suggested that the earliest title "Yabgu" was recorded in literary Chinese with regard to Kushan contexts with transliteration Xihou "e-khu (yephou)" (Chinese: 翖侯; literally: "United/Allied/Confederated Prince"). However, the Chinese does not make clear whether the title was the one bestowed on foreign leaders or rather a descriptive title indicating that they were allied, or united.
The Chinese word sihou (<*xiap-g’u) is a title. The second part of this compound, hou (<g’u), meant a title of second hereditary noble of the five upper classes. Sihou (<*xiap-g’u) corresponds to the title yavugo on the Kushan (Ch. Uechji) coins from Kabulistan, and yabgu of the ancient Türkic monuments [Hirth F. "Nachworte zur Inschrift des Tonjukuk" // ATIM, 2. Folge. StPb. 1899, p. 48-50]. This title is first of all a Kushan title, also deemed to be "true Tocharian" title. In the 11 BC the Chinese Han captured a Kushan from the Hunnu state, who was a "chancellor" (Ch. sijan) with the title yabgu (sihou). After 4 years he returned to the Hunnu shanyu, who gave him his former post of a «second [after Shanyu] person in the state", and retained the title yabgu (sihou). The bearer of this high title did not belong to the Hunnu dynastic line, well-known and described in detail in the sources. Probably, he was a member of the numerous Kushan (Uechji) autonomous diasporas in the Hunnu confederation. This history suggests, that in the Usun state Butszü-sihou, who saved the life of a baby Gunmo in the 160es BC, also was an yabgu.
It remains unclear whether the title indicates an alliance with the Chinese or simply with each other. A few scholars, such as Sims-Williams considered the Turkic "Yabgu" to be originally derived from the Chinese "Xihou". Another theory postulalates a Sogdian origin for both titles, "Yabgu" and "Shad". The rulers of some Sogdian principalities are known to have title "Ikhshid".
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