Kaiyuan Tongbao

The Kaiyuan Tongbao (開元通寶) was the standard bronze cash coin of the Tang dynasty of China, first cast in 621 CE under the dynasty's founding emperor, Gaozu. Round with a square central hole, it replaced the long-lived Wu Zhu and set the pattern for Chinese and wider East Asian coins for well over a thousand years.

Its four-character inscription is usually translated as "inaugural circulating treasure", marking a new beginning for the currency rather than naming a reign period. The coin was cast, not struck, and was produced in very large numbers by government foundries across the empire.

Before the Tang, Chinese money had been dominated by coins whose inscriptions stated their weight, a tradition running from the Ban Liang of the Qin through the Wu Zhu of the Han and later dynasties. The centuries of division that followed the fall of the Han left a confused mixture of light, worn and privately cast pieces in circulation, and the reunifying Sui dynasty had only partly restored order.

The Tang authorities broke with the weight-naming tradition. The new coin carried no weight in its legend; instead its value rested on a fixed official standard and on state backing. The calligraphy of the original inscription is traditionally attributed to the scholar-official Ouyang Xun. Because the characters can be read top-bottom-right-left or clockwise, the coin has also been called "Kaitong Yuanbao" in some sources.

The Kaiyuan Tongbao was cast in copper alloy using moulds and finished by hand. Its standard weight became so well known that it reshaped Chinese measures: ten coins were reckoned to weigh one liang (tael), and the word qian, meaning "coin", came to be used for a tenth of a liang. This link between a coin and a unit of weight shows how closely the coin served as both a unit of account and a physical measure.

Many specimens carry small marks on the reverse, most famously a crescent. A popular legend links the crescent to the fingernail of an empress pressing into a wax model, but the true origin of these marks is uncertain. Coins were strung together in strings of nominally 1,000 for larger payments.

Coins never met the full needs of the Tang economy. Bolts of silk and quantities of grain also served as media of exchange and as forms of tax payment, and the state regularly faced shortages of copper. Twice the court tried to issue larger coins at inflated face values: the Qianfeng Quanbao of 666, valued at ten ordinary coins, was soon withdrawn, and the Qianyuan Zhongbao introduced in 758 during the An Lushan rebellion helped fuel inflation and widespread illicit casting. Both episodes show the problems of overvalued token coinage that Gresham's law later described.

In the 840s, during the persecution of Buddhism under Emperor Wuzong, bronze from confiscated temple bells and statues was melted down and recast. These "Huichang" Kaiyuan coins often carry a single character on the reverse naming the casting place, an early form of mint mark.

The difficulty of moving heavy strings of coin over long distances also encouraged a Tang innovation known as "flying cash" (feiqian) in the early 9th century: merchants deposited coin in the capital and drew it out in the provinces against a certificate. This was a form of remittance comparable to a bill of exchange and is often seen as a forerunner of the paper money later issued under the Song dynasty.

The Kaiyuan Tongbao circulated for the whole Tang period and long afterwards, and several of the short-lived states of the Five Dynasties and Ten Kingdoms era cast their own versions. It served as the model for Japan's first official coinage, the Wadō kaichin of 708, and for early coins in Korea and Vietnam. Specimens and local imitations have also been found along the Silk Road in Central Asia.

Later Chinese coins generally followed its form: a round coin with a square hole carrying four characters, though from the Song onwards most legends named the reign period. Earlier Chinese forms such as knife money, spade money and the round huanqian of the Warring States had already given way to this shape, and the Tang version fixed it as the standard. The modern Chinese yuan stands at the far end of this long tradition.

• Tang dynasty – Encyclopaedia Britannica
• The British Museum
• American Numismatic Society