Yabghu or Yabgu 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 title had also been borne by Turkic princes in the upper Oxus region in post-Hephthalite times. The position of Yabgu was traditionally given to the second highest member of a ruling clan, 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. 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.
Etymology
There are at least five theories among recent literature regarding the origin of yabgu.
Others suggest that the word is a derivation of the early Turkic davgu,
A few scholars, such as Sims-Williams considered that Turkic languages had derived the wordyabgu from the Chinese "xihou". Conversely, Friedrich Hirth suggested that yabgu was translated into literary Chinese, with regard to Kushan and Turkic contexts, as Sihou. Other sources render this as Xihou . It was equivalent to the title yavugo found on Kushan coins from Kabul, and the yabgu on ancient Turkic monuments. The second part of this compound Chinese word, hou, referred to the second-ranking of five hereditary noble ranks. Chinese sources do not make clear whether the title was a descriptive term used only in reference to foreign leaders, or whether it indicated an ally or subject of a Chinese empire.
Another theory postulalates a Sogdian origin for both titles, "Yabgu" and "Shad". The rulers of some Sogdian principalities are known to have title "Ikhshid".
Yury Zuev considered Yabgu to be a "true Tocharian" title.
Friedrich Hirth has successfully compared the transcription sihou with a title yavugo on the Yuezhi-Kushan coins from Kabulistan and yabγu of the ancient Turkic monuments. This title is first of all an Yuezhi title, and it is a "true Tocharian" title. This originally Yuezhi royal title appears on the coins of their rulers as IAPGU/yavuga and it came to the Xiongnu from the Yuezhi. The title xihou corresponds in the pronunciation to what would later become the Turkic title yubgu. In the 11 BC an Yuezhi from the Xiongnu state fell in the Han captivity, he was a "chancellor" with the title sihou. After 4 years he returned to the Xiongnu chanyu. Chanyu gave him his former post of a "second man in the state" and retained the title sihou. The bearer of this high title did not belong to the Xiongnu dynastic line, but he was a member of the numerous Yuezhi autonomous diasporas in the Xiongnu confederation. This history suggests that in the Wusun state Butszü-sihou also was a yabgu. Among the Turks, the title yabgu gained a new lease of life. In the Turkish inscriptions of Mongolia, it refers to a noble ranking immediately after the qagan. Kuyan/gayan was a "common Yuezhi symbol for a terrestrial embodiment for the Moon and Milky Way". The myth about Milky Way Kagan found some new aspects among Turks and Mongols but the essence remained the same.
