on foreign trade and silver both entered and left China in large numbers, the market exchange rate between silver and copper changed drastically over time and tended to fluctuate, furthermore this exchange rate also varied from region to region. Services specialising in money exchanges, known as money-changers, developed in this currency system, and the exchange normally took place in commercial centres and trade ports where different trades were frequently carried out. The currency system in China during the Qing dynasty is sometimes called a "parallel bimetallic system", to distinguish it from the more conventional model of a bimetallic system. The term "parallel bimetallic system" is given to this system because it functioned more like a form of coexistence of "two currency systems, each using a different metal" than an actual bimetallic system. Also unlike real bimetallism in other countries, the actual exchange ratio between the two different metal currencies was not actually fixed; the exchange ratio tended to vary depending on time and place. Imperial government produced silver coinages Government produced silver coins during the Qianlong era During the reign of the Qianlong Emperor commemorative silver coins with the portrait of the Tibetan Panchen Lama are known to have been produced. Government produced silver coins during the Daoguang era During the reign of the Daoguang Emperor several attempts were made in China for the native production of government backed silver coinage, the first of such attempts were tried in the year 1821. Machine-struck Chinese silver coins were known to have been first produced in the year 1822, by the modern Jilin Arsenal Mint (吉林機器局). These early milled silver coins were known as the Changpingliang (廠平兩, literally "factory tael") and only had the denomination of one tael. These silver Changpingliang were not manufactured in any high numbers and are consequently very rare today. Other models of modern silver coinages, which are known as ban (板), that were known to have been produced in the cities of Guangzhou, Fuzhou, Hangzhou, Suzhou, Wuxi, and Jiangxi. The models of milled silver coins produced in Wuxi are known as xiban (錫板) and the ones produced in Jiangxi are known as tuban (土板). There were also the models known as Wuzhuang (吳莊) and Xingzhuang (行莊). Another early attempt at creating a native government-produced silver Chinese coinage was made by Lin Zexu, he created a system of silver coinages known as the Yinbing (銀餅, literally "Silver cakes")