一口通商
Legal trade in the South China Sea was resumed in 1727, but the East India Company's discovery that the prices and duties at Ningbo were both much lower than those at Guangzhou prompted them to begin shifting their trade north from 1755 to 1757. The Qianlong Emperor's attempt to discourage this through higher fees failed; in the winter of 1757, he declared that—effective the next year—Guangzhou (then romanized as "Canton") was to be the only Chinese port permitted to foreign traders, beginning the Canton System, with its Cohong and Thirteen Factories. Chinese merchants trading with foreigners, on the other hand, were not affected by any of these regulations.