What is "fanqie"

In traditional Chinese lexicography, fǎnqiè or fan-chieh is a method to indicate the pronunciation of a monosyllabic character by using two other characters, one with the same initial consonant as the desired syllable and one with the same rest of the syllable (the final). The method was introduced in the 3rd century AD and used in dictionaries and commentaries on the classics until the early 20th century.