The retrieval number of books generally includes internal number and book number. The internal numbers are classified according to the standard books in China Library Classification, and the types of books are indicated by letters, as follows: A Marxism-Leninism, Mao Zedong Thought, Deng Xiaoping Theory, B Philosophy, C Social Science, D Politics, Law, E Military, F Economy, G Culture, Science, Education, Sports, H Language, and so on. Introduction to natural science, O mathematical chemistry, P astronomy, Earth science, Q biological science, R medicine, health, S agricultural science, T industrial technology, U transportation, V aviation, aerospace, X environmental science, labor protection science, Z comprehensive books.