Each book also has a book number that designates its exact place on the shelf relative to other books on the same subject. The book number is usually based on the author's last name, so that books are arranged alphabetically by author within each subject classification. The subject classification system makes it possible to shelve all books on a specific subject in one place, and to shelve books on related subject in one area of the library. Every subject class has an underlying order that you will discover with familiarity. It may have occurred to you that if we use classification numbers to shelve books by subject that there is no need for a library catalog.
Many books cover more than one discrete subject, but they can have only one call number.
The call number will include the LC classification number that most closely matches its major subject or general subject area. To help you find multi-subject or interdisciplinary materials, the library catalog record includes as many subject headings as necessary to describe the subject content of the book. Let's look at a real example. The catalog record shown below is for an edition of Hamlet edited by Harold Jenkins, published in 1982. Another look at subject classification If the Shakespeare example did not clarify the effect of subject classification on shelf arrangement, here is another example.