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The four-corner method or four-corner system (simplified Chinese: 四角号码检字法; traditional Chinese: 四角號碼檢字法; pinyin: sì jiǎo hàomǎ jiǎnzì fǎ; lit. 'four corner code lookup-character method') 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 since been 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.