Wuzhu coin (3.28 g.) issued by the Han emperor Xuandi (73-47 BCE). Wuzhu coins were first issued in 118 BCE, and were eventually declared obsolete in 621 CE. The inscription consists of two characters: wu (meaning "five") to the right of the square hole; zhu (meaning "grain" - a weight term) to the left of the square hole. The inscription wuzhu means "5-grain", and implies a weight. However, documentary and textual evidence indicates that the wuzhu coin circulated with a value of "1", written in units of qian ("coin"). In this way, 100 wuzhu coins would be written as "100 qian".
Source: British Museum 1982-6-28-2. Used by permission.