Character amnesia
It is difficult to establish exactly how many different Chinese characters are in use today; the new HSK, a widely used proficiency test for Standard Chinese as a second language, tests over 2,600 different characters, whilst in 1988 the People's Republic of China published the "List of Commonly Used Characters in Modern Chinese" which contains 7,000 different characters. In Japan, where a smaller set of characters are in general usage, the Japanese Ministry of Education prescribes the teaching of 2,136 kanji in primary and secondary school in a list called the jōyō kanji, meaning "regular-use Chinese characters".
A Book from the Sky
The book is composed using a set of 4,000 characters, as this is roughly the number of characters in common usage in modern written Chinese. These characters were designed on the basis of the Kangxi radicals, so that “in terms of density of strokes and frequency of occurrence, they… appear, on the page, to be real characters“. In addition to these, page and fascicle numbers were indicated using tally marks based on the Chinese character 正.