Idu (이두, hanja : , meaning official's reading) is an archaic writing system that represents the Korean language using hanja. The term "idu" is used in two senses. It may refer to various systems of representing Korean phonology through Chinese characters called hanja, which were used from the early Three Kingdoms to Joseon periods. In this sense it includes hyangchal and gugyeol writing, as well as the narrower sense of "idu". The narrower sense refers solely to the system developed in the Goryeo period (918–1392), and first referred to by name in the Jewang Ungi.