Khong Tayiji derives from Chinese Huangtaizi (皇太子; crown prince). At first it also meant crown prince in Mongolian. It was given only to descentans of Chinggis Khan. In the Mongol tradition, a khan was unable to appoint the successor, but he was elected in the khuriltai after the khan's death. Khubilai Khan installed his second son Chingem as Crown Prince for the first time. After Chingem died in 1284, the position was took over by his third son Temür in 1293. However, the Crown Prince was not the definite successor. He was also elected in a khuriltai.
Khong Tayiji became sub-Khan since Altan Khan of the Tümed tümen installed Khong Tayiji as assistant Khad. This convention spread throughout the Mongols.
In 1630s the head of the Jüün Ghar was given the title of Baatur Khong Tayiji by the Dalai Lama. Since then Khong Tayiji was took over by the Jüün Ghar chiefs. As the Jüün Ghar got stronger, the title became higher. In the mid-18th century it went beyond Khan among the Oyirad because too many chiefs were appointed Khad by the Qing Dynasty.
See also: Jasagh