How did the idu system use Chinese characters to write Korean?

The idu system, developed by the 7th century, was an early method used by Koreans to represent their language using Chinese characters. Rather than relying solely on the original meaning of the characters, idu utilized specific Chinese characters for their phonetic value to transcribe Korean particles of speech and inflectional endings.1

This system allowed for rough transliterations of Chinese texts into a form that reflected Korean grammar. It served as a precursor to more advanced transcription systems, such as hyangch’al, which enabled the writing of entire Korean sentences using Chinese characters, and kugyŏl, which inserted abridged characters into texts to denote grammatical elements. These systems were essential for writing in Korean before the invention of the Hangul alphabet in the 15th century.12