Idu (Korean: 이두; Hanja: 吏讀; lit. 'official's reading') was a writing system developed during the Three Kingdoms period of Korea (57 BC-668 AD) to write the Korean language using Chinese characters ("hanja"). It used Hanja to represent both native Korean words and grammatical morphemes as well as Chinese loanwords. The script, which was developed by Buddhist monks, made it possible to record Korean words through their equivalent meaning or sound in Chinese. It was used primarily to write official documents and the imperial examinations from 958 AD-1894 AD. The term idu may refer to various systems of representing Korean phonology through hanja, which were used from the early Three Kingdoms to Joseon periods. In this sense, it includes hyangchal, the local writing system used to write vernacular poetry and gugyeol writing. Its narrow sense only refers to idu proper or the system developed in the Goryeo (918–1392), and first referred to by name in the Jewang ungi. Source: Wikipedia (en)