Cover refers to the outer layer of books and periodicals. Sometimes it refers to the first side with the title, author or editor, publisher name, etc. XI in Ba Jin's Home: "My grandfather just gave me a thread-bound book, and I read the title of the white label on the cover:" Mr. Liu taught me to be filial to my parents and abstain from lewdness ",and I felt a headache."

The back cover, also known as the fourth cover and the bottom cover, refers to the bottom of the book cover, which is an important part of the book and an extension, supplement, summary or emphasis of the cover and spine. The back cover and the cover are closely related, complementary and indispensable.