Four corner method

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The Four corner method is a method of encoding Chinese characters using four numerical digits per character (in some situations, an additional digit is used). It began as a method of indexing Chinese characters in dictionaries, and was popular before the wide spread use of pinyin. It was then developed as an input method for computer.

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:

横一垂二三点捺
叉四插五方框六
七角八八九是小
点下有横变零头

In short, the number 1 represents a horizontal stroke, 2 represents a vertical or diagonal stroke, 3 a dot stroke, 4 two strokes in a cross shape, 5 three or more strokes in which one stroke intersects all others, 6 a box-shape, 7 where a stroke turns a corner, 8 the shape of the Chinese character 八 and its inverted form, and 9 is used for the shape of the Chinese character 小 and its inverted form. Zero is used where there is either nothing in a corner, the part in a corner is already represented by a previous corner, or where a corner has a dot stroke followed by a horizontal stroke.

Several other notes:

A single stroke can be represented in more than one corner, as is the case with many curly strokes. (eg. the code for 乙 is 1771)
If the character is fenced by 囗, 門(门), or 鬥, the lower corners are used to denote what is inside the radical, instead of 00 for 囗 or 22 for the others. (eg. the code for 回 is 6060)

Its use is not common.