The Chinese word "嘧啶" is formed solely for phonetic transliteration of the English chemical term "pyrimidine", with each character constructed by combining the radical "口" (indicating a sound-related borrowing) with a phonetic component: "嘧" uses "密" (mì) for its sound, and "啶" uses "定" (dìng) for its sound, together approximating the syllables of the source word.