Book classification, also called book classification thesaurus, is classified according to the contents, forms, genres and readers' uses. Under the guidance of certain philosophical thoughts, the books of various disciplines are divided into several categories according to their subject contents by using the principle of knowledge classification and logical methods. Each category is divided into many subclasses, and each subclass is divided into molecular subclasses. 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.