There are many small cabinets called index card cabinets at the front desk of the library, which are divided into several categories for us to retrieve books. For example, you can use the stroke order or pinyin order of the title of the book as the basis for your search, or you can query the book you want from the stroke order or pinyin order of the author's name. Anyway, there are many retrieval methods, but it is obvious that the books in the stacks are not arranged in order in these cabinets-although this can be done in theory, in fact all the books are pasted on the back by hand. The index card does not indicate which shelf the book is placed on, just this specific number. The librarian will return the books he wants to the readers according to this number.