In order to maintain national unity, rule by custom has become a policy of national governance in Qing dynasty after Xinjiang is united in the mid-18th century. Such is the case with taxation. In northern Xinjiang and areas with close contact with the mainland, the taxation on land is implemented in accordance with former practices for the local Uygur people. Taxation has undergone reforms through to the 19th century when it it substantially optimized by foreign experiences.