This was the main coin issued by the Tang. It was cast for most of the dynasty, a period of nearly 300 years. It was first issued by the Emperor Gao Zu in the autumn of the 4th year of the Wu De period (August 621). Its diameter was to be 8 fen. The weight was set at 2.4 zhu, ten to the liang. 1,000 coins weighed 6 jin 4 liang. The legend was written by the famous calligrapher Ouyang Xun in a much admired mixture of the Bafen and Li (official or clerkly) styles of writing. This is the first to include the phrase tong bao, used on many subsequent coins. The inscription was used by other regimes in later periods; such coins can be distinguished from Tang coins by their workmanship. Minting and copper extraction were centrally controlled, and private casting was punishable by death. For the first time we find regulations giving the prescribed coinage alloy ¨C 83% copper, 15% lead, and 2% tin. Previously the percentages used seem to have been on an ad hoc basis. Actual analyses show rather less copper than this.
A crescent-shaped mark is often found on the reverse of Kai Yuans. The legend is that the Empress Wende inadvertently stuck one of her fingernails in a wax model of the coin when it was first presented to her, and the resulting mark was reverentially retained. Other imperial ladies have also been proposed as the source of these nail marks, especially the Imperial Consort Yang. Peng explores the possibility of a foreign source for them. More prosaically, they appear to be a control system operated by the mint workers.
At first, mints were set up in Luoyang in Henan, and also in Peking, Chengdu, Bingzhou (Taiyuan in Shanxi), and then Guilin in Guangxi. Minting rights were also granted to some princes and officials. By 660, deterioration of the coinage due to forgery had become a problem. The regulations were reaffirmed in 718, and forgeries suppressed. In 737, the first commissioner with overall responsibility for casting was appointed. In 739, ten mints were recorded, with a total of 89 furnaces casting some 327,000 strings of cash a year. 123 liang of metal were needed to produce a string of coins weighing 100 liang. In the late 740s, skilled artisans were employed for casting, rather than conscripted peasants. Despite these measures, the coinage continued to deteriorate. In 808, a ban on hoarding coins was proclaimed. This was repeated in 817. Regardless of the rank of a person, they could not hold more than 5,000 strings of cash. Cash balances exceeding this amount had to be expended within two months to purchase goods. This was an attempt to compensate for the lack of cash in circulation. By 834, mint output had fallen to 100,000 strings a year, mainly due to the shortage of copper. Forgeries using lead and tin alloys were produced.
In 845, in the Huichang period, the Emperor Wu Zong, a fervent follower of Taoism, destroyed the Buddhist monasteries and used the copper bells, gongs, incense burners and statues to cast coins in various localities. These local mints were under the control of the provincial governors. The New Tang History states that Li Shen, governor of Huainan province, requested that the empire might cast coins bearing the name of the prefecture in which they were cast, and this was agreed. These coins with mint names on the reverses, known as Huichang Kai Yuans, are of poor workmanship and size compared with the early Kai Yuans. However, when Xuanzong ascended to the throne the next year, this policy was reversed, and the new coins were recast to make Buddhist statues.
Archaeological discoveries have assisted numismatists in dating various varieties of the Kai Yuan more closely.

Tang Dynasty (618 - 907 AD)