Wu Zhu

The wu zhu ("five zhu") is a round bronze coin with a square hole, first issued by the Han dynasty in 118 BCE under Emperor Wu. Named after its nominal weight of five zhu, a unit of roughly 0.65 grams, it became the standard coin of China for around seven centuries and is one of the longest-lived coin types in world history.

The wu zhu replaced the lighter, irregular ban liang and gave Chinese coinage a stable, recognisable form. It remained in production, with interruptions, under the Han and many later dynasties until the Tang dynasty introduced the kaiyuan tongbao in 621 CE.

In the early Han period the bronze currency was in disorder. The ban liang had been reduced in weight several times, private casting had been permitted, and coins of very different sizes and quality circulated together. Emperor Wu (r. 141–87 BCE) also needed revenue for military campaigns, and the court tried several monetary experiments, including high-value token issues, before settling on the wu zhu.

The wu zhu was introduced in 118 BCE. At first coins were cast by commanderies (local administrations) as well as by the central government, and counterfeiting was a serious problem. From 113 BCE the court banned all casting outside central control and placed production under offices in the Shanglin park near the capital, Chang'an. Older coins were withdrawn and melted down.

Centralised production allowed much more consistent weight and quality. The official history of the Han records that more than 28 billion wu zhu coins were cast between 118 BCE and the end of the first century BCE, an indication of the scale of the Han money supply.

The wu zhu is round, with a square hole and the two characters wu and zhu on either side of the hole. Unlike the ban liang, it has a raised outer rim, which protected the coin from wear and made it harder to clip metal from the edge. The reverse normally has a raised rim around the hole. Small marks, such as bars or dots near the hole, appear on some issues and help numismatists distinguish periods and mints. Actual weights vary, but well-made Han pieces are typically around three to four grams.

The usurper Wang Mang, who founded the short-lived Xin dynasty (9–23 CE), abolished the wu zhu and introduced a complex system of coins, spade-shaped and knife-shaped pieces, and other forms of value. His reforms caused confusion and loss of confidence. After the Han was restored, Emperor Guangwu of the Eastern Han reinstated the wu zhu, which then remained the standard coin for the rest of the dynasty.

Later states during the period of division between Han and Tang, including several of the Northern and Southern dynasties and the Sui, continued to cast coins with the wu zhu inscription. Quality varied: some issues were well made, while others were light or debased, reflecting fiscal pressure and periods of currency debasement.

The wu zhu served as the everyday cash of the Han empire, used for taxes, wages, rents and trade. Han tax obligations were partly paid in coin, which drew money into even rural areas. Wu zhu coins have been found along the trade routes linking China with Central Asia, and the coin influenced early coinages beyond China's borders.

The coin's long life made the round, rimmed, square-holed bronze coin the model for East Asian money. The Tang kaiyuan tongbao of 621 CE replaced the weight-based name with an inscription describing the coin as circulating treasure, but kept the same basic form, which continued through Song dynasty coinage and into the Qing period. Together with the earlier huanqian, spade money and knife money, the wu zhu traces the path from regional state monies to a single imperial coin and stands as one of the cornerstones of Chinese monetary history.

• British Museum – Collection online
• American Numismatic Society