Prior to the 13th century, Tai kingdoms had existed in the northern highlands including the Ngoenyang Kingdom of the Tai Yuan people (centered on Chiang Saen and the predecessor of the Lanna Kingdom), and the Heokam Kingdom of the Tai Lue people (centered on Chiang Hung, modern Jinghong in China). Sukhothai had been a trade center and part of Lavo (present day Lopburi), which was under the domination of the Khmer Empire. The migration of Tai people into the upper Chao Phraya valley was somewhat gradual.