CJK characters
Chinese is written almost exclusively in Chinese characters. It requires over 3,000 characters for general literacy, but up to 40,000 characters for reasonably complete coverage. Japanese uses fewer characters — general literacy in Japanese can be expected with 1,945 characters. The use of Chinese characters in Korea is becoming increasingly rare, although idiosyncratic use of Chinese characters in proper names requires knowledge (and therefore availability) of many more characters. However, even today, students in South Korea are taught 1,800 characters.
Xu Shen
In the postface, Xu gives a brief account of the history of writing. According to legend, Chinese characters were first invented by Cang Jie, who was inspired by footprints to create a system of signs that refer to the natural world. These original graphs could then be combined to make meaningful characters with referents distinct from their component graphs. This was important for Xu Shen, who emphasized the productivity, even fertility, of Chinese writing. Despite the novel meanings of compound characters, Xu Shen believed that true understanding of composite characters was contingent on an understanding of their components. Providing clear explanation of these relationships lay at the heart of his motivation.
Central to Xu Shen's thought is the contrast between "wen" (文 patterns) and "zi" (字 characters), indeed these contrasting categories of graphs receive separate mention in the work's title. Even today, there is disagreement over the exact definitions of the two terms. The Song dynasty scholar Zheng Qiao (郑樵) first presented the interpretation that "wen" and "zi" are the difference between non-compound and compound characters. More recently, other scholars, such as Françoise Bottéro (2002), have argued that the addition of a specifically phonetic element (and thus not simply compounding) marks the principal difference between "wen" and "zi". From this original binary contrast, Xu Shen formally delineated for the first time the six categories (六书) of Chinese characters.
To organize the thousands of headwords, Xu Shen established 540 radicals, and ordered them from least to greatest complexity. Each radical then headed its own group, which in turn subsumed all composite characters which incorporated the specific radical. The total number of 540 has a cosmological weight, as it can be derived from the product of 6, 9, and 10. 6 and 9 are the numbers of Yin and Yang, and 10 a number signifying completion. This number symbolizes the exhaustive scope of the dictionary.