For the second Emperor of the Qing dynasty, see Hong Taiji.
Khong Tayiji (ᠬᠤᠨ ᠲᠠᠶᠢᠵᠢ，Mongolian: хун тайж) also spelled Qong Tayiji; in Manchu: Hong Taiji) is a title of the Mongols.
Khong Tayiji derives from ChineseHuangtaizi (皇太子; crown prince). At first it also meant crown prince in Mongolian. It was originally given only to descendants of Genghis Khan. In the Mongol tradition, a khan was unable to appoint the successor, instead the successor was elected in the kurultai after the khan's death. However Kublai Khan (who founded the Yuan Dynasty) broke this tradition, and installed his second son Zhenjin (Chingem) as Crown Prince. After Chingem died in 1286, the seal of Crown prince was passed to Chingem's third son Temür in 1293. However, Temür was never formally appointed as the Crown Prince and still not the definite successor. He was only confirmed as successor in a kurultai held after Kublai's death.
The Khong Tayiji became sub-Khan when Altan Khan of the Tümed tümen installed the Khong Tayiji as assistant khan. This convention of a sub-khan spread throughout the Mongol world.