Four-Corner Method - How IT Works

How It Works

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:

Traditional Simplified Pinyin Meaning

一橫二垂三點捺，
點下帶橫變零頭，
叉四插五方塊六，
七角八八小是九。

一横二垂三点捺，
点下带横变零头，
叉四插五方块六，
七角八八小是九。

Yī héng, èr chuí, sān diǎn, nà;
Diǎn xià dài héng, biàn líng tou;
Chǎ sì, chā wǔ, fāng kuài liù;
Qī jiǎo, bā ba, xiǎo shì jiǔ.

1 for horizontal, 2 vertical, 3 is a dot;
a dot over a horizontal, or already another corner, is 0;
crossing is 4, crossing more than one is 5, a box is 6;
7 for a corner, 八 (shape of '8' character) is 8, and 小 is 9.

In the 1950s, lexicographers in the People's Republic of China changed the poem somewhat in order to avoid association with Hu Shi, although the contents remain generally unchanged. For various reasons, his name was "unmentionable" at the time the new version was composed. The 1950s version is as follows:

Traditional Simplified Pinyin Meaning

横一垂二三點捺，
叉四插五方框六，
七角八八九是小，
點下有横變零頭。

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。

Héng yī, chuí èr, sān diǎn, nà;
Chǎ sì, chā wǔ, fāng kuàng liù;
Qī jiǎo, bā ba, jiǔ shì xiǎo;
Diǎn xià yǒu héng, biàn líng tóu.

horizontal is 1, vertical 2, 3 is a dot;
crossing is 4, crossing more than one is 5, a box is 6;
7 for a corner, 8 for 八 (shape of '8' character), 9 is 小;
and a dot over a horizontal, or already another corner is 0.

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 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. (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.

Read more about this topic:  Four-Corner Method

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