Call number, also called call number, is the code used by library book shelving and the main basis for literature borrowing and collection inventory. Generally, it consists of several groups of numbers arranged by branches, which are often printed on the upper left corner of the catalog card, on the book label below the spine, and above the title page or back cover of the book. A call number can only represent one kind of book. ?

Because the shelving methods of library books can basically be divided into two categories: classified shelving and formal shelving. Therefore, call numbers can basically be divided into two categories: classified call numbers and formal call numbers.

Book classification, also known as book classification thesaurus, is based on the content, form, genre and readers' use of books, under the guidance of certain philosophical thoughts, using the principle of knowledge classification and adopting logical methods.

Books of all disciplines are divided into several categories according to the subject content, and each category is divided into many subcategories, and each subcategory is subdivided into molecular subcategories. Finally, every kind of book can be divided into certain categories, and each category has a category number. Classified thesaurus is a collection of hierarchical classification numbers and categories.

The China Library Classification adopts the mixed numbering of Chinese phonetic alphabet and Arabic numerals, with one letter representing a major category, reflecting the alphabetical order of major categories, and the disciplines subdivided under major categories are composed of Arabic numerals. In order to adapt to the development of industrial technology and the classification of this kind of literature, the two-letter classification of industrial technology is adopted.