Tang Dynasty
Exterior diameter: 2.6cm

During the fourth year of the Reign of Emperor Gaozu in the Tang Dynasty (621 A.D.), "Kaiyuan Tongbao" was used soon after the abolition of the circulation of "Wuzhuqian". "Tongbao" means "the treasure (i.e. coin) which could pass freely," while "Kaiyuan" is the name of the reign in which the coin was made. Kaiyuan coins were the major currency of the Tang Dynasty. Ten Kaiyuan coins weigh one ounce. Such reform brought great influence over the measuring system of ancient China. As far as the scale and weight of "Kaiyuan Tongbao" is concerned, it had become, later on, the model for the coins produced by Ming Dynasty rulers. One ounce of weight in the later generations equals the total weight of one cent during the Kaiyuan period. It had thus become an important factor in creating the decimal system.