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The Four Corner code is a method of describing and indexing Han and Han-derived characters based on their visual layout. It was developed by Wang Yun-Wuu in the 1920s and became popular for dictionary indexing of Chinese characters before the large-scale use of Pinyin. The code is comprised of four numbers, each describing a feature of the four corners of a character. An optional fifth number is sometimes used for disambiguation between similar characters.
This page provides a very basic overview of the four corner method. Please be aware that the four corner codes used in the KanjiDB database are not guaranteed to be error free, so searching by four corner codes should be used with caution.
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Character elements are divided into ten basic classes:
0 Lid 亠 1 Horizontal Line 一 2 Vertical Line ｜ 3 Dot 丶 4 Cross 十 5 Skewer キ 6 Box 口 7 Angle 厂 8 Hachi 八 9 Chiisai 小
The four digits of the code are obtained by classifying each corner of a character, starting from the top left in a Z-shape pattern:
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Some examples:
1) A single shape is only used once. If it fills several corners, it is counted as zero in subsequent corners. Multiple occurences of the same shape are counted normally.
Examples:
2) When the upper or lower half of a character consists of only one (single or composite) shape, it is, regardless of its position, counted as a left corner. The right corner is counted as zero.
Examples:
3) When there is no additional element to the four sides of the characters 口, 門, 鬥 (and sometimes 行), whatever is inside these characters is taken for the lower two corners.
Examples:
4) The analysis is based on the handwritten block-style (楷書 - kaisho) of characters. This can be slightly confusing in some instances. For example, 戸 is 3027, not 1027. The top stroke is treated as a 丶.
亠 - Lid Shape
When the horizontal line below a DOT shape (number 3) is connected to another stroke at its right-hand end (as in 宀戸, etc.) it is not counted as a LID (number 0) but as a DOT.
Examples:
口 - Box Shape
Characters such as 皿and 尸where one of the strokes of the square extends beyond it, are not considered to be box (number 6) shapes, but angles (number 7).
Examples:
厂 - Angle Shape
Only the cornered end of angle shapes (number 7) is counted as 7.
Examples:
八 - Hachi Shape
Strokes that cross other strokes are not counted as shape number 8 (八).
Examples:
小 - Chiisai Shape
Shapes resembling shape 9, but featuring two strokes in the middle (as in the top part of 業or 叢) or two strokes on one side (as in 水 or the bottom part of 慕) are not considered as 9 shapes.
Examples:
6) Some points to note when choosing corners:
When a corner is occupied by more than one independent or parallel strokes, the one that extend furthest to the left or right is taken as the corner, regardless of how high or low it is.
Examples:
If there is another shape above (or, at the bottom of the character, below) the leftmost or rightmost stroke of a character, that shape is given preference and is taken as the corner.
Examples:
When two composite stroke shapes are interwoven and each could be regarded as a corner, the shape that is higher is taken as the upper corner, and the lower stroke as lower corner.
When a stroke that slopes downwards to the left or right is supported by another stroke, the latter is taken as the corner.
Examples:
A left slanting stroke on the upper left is taken for the left corner only; for the right corner one takes a stroke more to the right.
Examples:
7) In order to differentiate between several characters with the same code, an optional "fifth corner" is sometimes used. This is, loosely, a shape above the fourth corner which has not been used in any other shape.
In the KanjiDB database, fifth corners are denoted ABCD.E, and characters without a fifth corner have a default fifth corner of '0'.
Given the complicated nature of the four corner method, it isn't difficult to see why it has been displaced by Pinyin (phonetic romanized Mandarin) as the popular choice for Chinese speakers in indexing hanzi. It is no longer in common usage.