In traditional Chinese lexicography, fanqie (Chinese: 反切; pinyin: fǎnqiè; lit. "reverse-correspondence") is a method to indicate the pronunciation of a character by using two other characters, each giving part of the pronunciation.
Early dictionaries such as the Erya (3rd century BC) indicated the pronunciation of a character by the method of dúruò (讀若, "read as"), giving another character with the same pronunciation. The introduction of Buddhism to China around the first century brought Indian phonetic knowledge, which may have inspired the idea of fanqie. Sun Yan (孫炎), of the state of Wei during the Three Kingdoms period (220–280 AD), is generally considered to be the first to adopt fanqie in his Erya Yinyi (爾雅音義, "Sounds and Meanings of Erya").
In the fanqie method, a character's pronunciation is represented by two other characters. The onset (initial consonant) is represented by that of the first of the two characters (上字 "upper word", as Chinese was written vertically); the final (including the medial glide, the nuclear vowel and the coda) and the tone are represented by those of the second of the two characters (下字, "lower word"). For example, the character 東 is described by the formula 德紅切. The third character 切 indicates that this is a fanqie spelling, while the first two characters indicate the onset and final respectively. Thus the pronunciation of 東 [tuŋ] is given as the onset [t] of 德 [tək] with the final [uŋ] of 紅 [ɣuŋ], with the same tone as 紅.
The method was used by the Qieyun (切韻, "Cutting Rhymes"), a Chinese rhyme dictionary published in 601 AD during the Sui Dynasty. When classical Chinese poetry flowered during the Tang Dynasty, the Qieyun became the authoritative source for literary pronunciations and it repeatedly underwent revisions and enlargements, the most important of which was the Guangyun (1007–1008). Even after the more sophisticated rime table analysis was developed, fanqie continued to be used in dictionaries, including voluminous Kangxi Dictionary published in 1716.
Effects of sound change 
The fanqie described the pronunciations of characters in Middle Chinese, but the relationships have been obscured as the language evolved into the modern varieties over the last millennium and a half. Middle Chinese had four tones, and initial plosives and affricates could be voiced, aspirated or voiceless unaspirated. Syllables with voiced initials tended to be pronounced with a lower pitch, and by the late Tang Dynasty, each of the tones had split into two registers (traditionally known as yin and yang) conditioned by the initials. Voicing then disappeared in all dialects except the Wu group, with consonants becoming aspirated or unaspirated depending on the tone. The tones then underwent further mergers in various varieties of Chinese. Thus the changes in both the initial and the tone were each conditioned by the other, represented by different characters in the fanqie pair.
For example, the characters of formula 東 [tuŋ] = 德 [tək] + 紅 [ɣuŋ] are pronounced dōng, dé and hóng in modern Standard Chinese, so that the tones no longer match. This is because the voiceless initial [t] and the voiced initial [ɣ] condition different registers of the Middle Chinese level tone, yielding the first and second tones of the modern language. (The pinyin letter d represents the voiceless and unaspirated plosive [t].)
This effect sometimes led to a form of spelling pronunciation, as in an example cited by Chao Yuen Ren. The character 强 had two readings in Middle Chinese. It could be read [gjɑng] in the level tone, meaning "strong, powerful"; this developed regularly into the modern reading qiáng. It could also be read [gjɑng] in the rising tone, meaning "stubborn" or "forced". The regular development would be for the voiced initial [g] to condition the yang register of the rising tone, becoming the fourth tone of modern Chinese, and for the rising tone to condition an unaspirated initial. Thus we would expect jiàng, which indeed does occur in the sense "stubborn", but the character also has the unexpected pronunciation qiǎng for the sense "forced". Chao attributed this to the fanqie formula 强 = 其 [gi] (level tone) + 兩 [ljɑng] (rising tone) given in dictionaries. Here the first character is now pronounced qí, because in the level tone the voiced initial becomes aspirated, while the second character is now pronounced liǎng, because in the rising tone sonorants like [l] condition the yin register, leading to the modern third tone.
- G. Casacchia (2006). "Chinese Linguistic Tradition". Encyclopedia of Language & Linguistics. Second Edition. London: Elsevier.
- W. S.-Y. Wang and R. E. Asher (1994). "Chinese Linguistic Tradition". The Encyclopedia of Language and Linguistics. Oxford: Pergamon Press.
- 竺家寧。《聲韻學》。台北：五南圖書。1990。[in Traditional Chinese] [This book pointed out that fanqie's use appeared as early as Eastern Han]
- Wang Li (1980). 漢語史稿. ISBN 7-101-01553-0
- Norman, Jerry (1988). Chinese. Cambridge University Press. pp. 34–36, 52–54. ISBN 978-0-521-29653-3.
- Chao, Yuen Ren (1976). "What is correct Chinese?". Aspects of Chinese sociolinguistics: essays by Yuen Ren Chao. Stanford University Press. pp. 72–83. ISBN 978-0-8047-0909-5.