As the name implies, a substitute board is a board that replaces books. When selecting a book, insert the book board in the selected bookshelf. If the selected book is suitable, take out the book board and take it away with the book. If the selected book is not suitable, put the book back on the original shelf, retrieve the replacement board, and select another book on the shelf. The correct use of substitute board can greatly reduce the phenomenon of books being piled up and placed in disorder, and also reduce the time for readers to choose books.