Iu is an archaic writing system which 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, which were used from the 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, and first referred to by name in the Jewang Ungi.