[1][2] The pronunciation of each homophone group was indicated by a fǎnqiè formula, a pair of characters having respectively the same initial and final sound as the word being described.
Pairs of syllables that are not so distinguished are known as chongniu, and occur only with certain division-III finals and with labial, velar or laryngeal initials.
[12][13] The Middle Chinese notations of Li Fang-Kuei and William Baxter distinguish the chongniu-IV parts, spelt with both "j" and "i", from chongniu-III parts, spelt with only "j"; without any commitment to pronunciation:[10] Baxter identifies some other finals that behave like chongniu finals, but do not occur paired within Qieyun rhyme groups:[15] Each of the chongniu-IV finals falls within a single Old Chinese rhyme class, but the chongniu-III and non-chongniu parts of 支 zhī and 真 zhēn span two Old Chinese rhyme classes.
[17][18] It is, however, reflected in the choice of Chinese characters to represent Old Japanese syllables in the Man'yōgana system, particularly the kō/otsu distinction between i1 and i2 after velars and labials.
[25] Sino-Korean shows a palatal glide where division-IV chongniu finals follow velar or laryngeal initials.
[24] The nature of the distinction within Middle Chinese is disputed, with some scholars ascribing it to a medial and others to the main vowel.