Four Corner Method

The Four-Corner Method (simplified Chinese: 四角号码检字法; traditional Chinese: 四角號碼檢字法; pinyin: sì jiǎo hàomǎ jiǎnzì fǎ; literally: "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-Corner Method is also known as the Four-Corner System.

The four digits encode the shapes found in the four corners of the symbol, top-left to bottom-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 bottom-right if necessary.

The Four-Corner Method was invented in the 1920s by Wang Yunwu (王雲五), the editor in chief at Commercial Press Ltd., China. Its development was based mainly on contributions by the Russian scholar Otto Rosenberg in the early 20th century, as well as experiments by Lin Yutang and others. Its original purpose was to aid telegraphers in looking up Chinese telegraph code (CST) numbers in use at that time from long lists of characters. This was mentioned by Wang Yunwu in an introductory pamphlet called Sijiaohaoma Jianzifa in 1926. Introductory essays for this pamphlet were written by Cai Yuanpei and Hu Shi.

The four digits used to encode each character are chosen according to the "shape" of the four corners of each character, i.e. the upper left, upper right, lower left and lower right corners. The shapes can be memorized using a Chinese poem that Hu Shi composed, called Bihuahaoma Ge, as a "memory key" to the system:

...
Wikipedia