The Four-Corner Method or Four-Corner System () is a character-input method used for encoding Chinese characters into either a computer or a manual typewriter, using four or five numerical digits per character.
The four digits encode the shapes found in the four corners of the symbol, upper left to lower right. Although this does not uniquely identify a Chinese character, it leaves only a very short list of possibilities. A fifth digit can be added to describe an extra part above the lower right if necessary.
The four-corner method, in its three revisions, was supported by the Chinese state for a while, and is found in numerous older reference works and some still in publication. The small Kangorin Sino-Japanese Dictionary by Yoneyama had a four-corner index when it was introduced in the 1980s, but it has been since deleted. However, it is not in common usage today, although dictionaries using it are available. It is identified, in public opinion, with the time when many Chinese were illiterate and the language was not yet unified; more Chinese today use the dictionary to help them write, not read. But it is useful for scholars, clerks, editors, compilers, and especially for foreigners who read Chinese. In recent years it has achieved a new usage as a character input system for computers, generating very short lists to browse.
Origin
The Four-Corner Method was invented in the 1920s by Wang Yunwu, the editor in chief at Commercial Press Ltd., China. It was based on experiments by Lin Yutang and others. Its original purpose was to aid telegraphers in looking up Chinese telegraph code numbers in use at that time from long lists of characters. This was mentioned by Wang Yunwu in an introductory pamphlet called Four-Corner Method, published in 1926. Cai Yuanpei and Hu Shih wrote introductory essays for this pamphlet.
Mnemonics
The four digits used to encode each character are chosen according to the "shape" of the four corners of each character. In order, these corners are upper left, upper right, lower left and lower right. The shapes can be memorized using a poem composed by Hu Shih, called Bihuahaoma Ge (), as a "memory key" to the system:
In the 1950s, lexicographers in the People's Republic of China changed the poem somewhat in order to avoid association with Hu Shih, who had criticized the Chinese Communist Party, although the contents remain generally unchanged. The 1950s version is as follows:
Several other notes: A single stroke can be represented in more than one corner, as is the case with many curly strokes. (e.g. the code for 乙 is 1771) If the character is fenced by , (门), or , the lower corners are used to denote what is inside the radical, instead of 00 for 囗 or 22 for the others. (e.g. the code for 回 is 6060)
There have been scores, maybe hundreds, of such numerical and alpha-numerical systems proposed or popularized (such as Lin Yutang's "Instant Index", Trindex, Head-tail, Wang An's Sanjiahaoma, Halpern); some Chinese refer to these generically as sijiaohaoma (after the original pamphlet) though this is not correct.
Versions
Over time, the Four-Corner Method has gone through some changes.
First Version
The first (revised) version was published in Shanghai in 1928. It was quickly adopted and popularized as a method for (among other things): Arranging and indexing Chinese characters in dictionaries Indexing Chinese classical and modern books, libraries, hospital and police records Chinese typewriters Military code making (for handling the characters quickly)
The Wang Yun-wu Da Cidian of 1928 was remarkable for its time, and although the pronunciations were very much in line with today's Standard Chinese, the lack of a phonetic index diminished its overall usefulness. The northern Mandarin pronunciations were given in Gwoyeu Romatzyh, a romanization system devised by linguist Zhao Yuanren, as well as in Mandarin Phonetic System (MPS or Bopomofo) characters with a dotted corner for tone. It also delineated parts of speech, and all compounds were listed by the four-corner method as well.
The famed lexicographer and editor of Ciyuan, Lu Erkui, as well as other lexicographers, became early proponents of the Four-Corner Method. By 1931, it was used extensively by the Commercial Press to index virtually all classical reference works and collections of China, such as the Pei Wen Yun Fu and Complete Library of the Four Treasuries, as well as many modern ones.
