The idu script used hanja, along with special symbols to indicate Korean verb endings and other grammatical markers that were different in Korean from Chinese. This made both the meaning and pronunciation difficult to parse, and was one reason why the system was gradually abandoned, to be replaced with hangul, after the invention of such in the 15th century. In this respect, it faced problems analogous to those that confronted early efforts to represent the Japanese language with kanji, due to grammatical differences between these languages and Chinese. In Japan the early use of Chinese characters for Japanese grammar was in man'yōgana, which was replaced by kana, the Japanese syllabic script.
Characters were selected for idu based on their Korean sound, their adapted Korean sound, or their meaning, and some were given a completely new sound and meaning. At the same time, 150 new Korean characters were invented, mainly for names of people and places. Idu system was used mainly by members of the Jungin class.