To find the book itself, you need three essential pieces of information, all of which can be found in the book's record:
1. Is the book available?
2. Which section is it in?
3. Where is it on the shelf?
This is a picture of a bookshelf in De Paul Library.
See those letters and numbers on the spine? Those are call numbers; they tell us where on the shelf the book should go. Think of a call number as a book's street address.
Our library uses call numbers from the Library of Congress Classification System -- not Dewey Decimal like public and school libraries.
While it's important to be able to find a book's physical location, the most important job of a call number is to group books by subject. All those letters and numbers are a kind of code for what the book is about. What this means is that once you've looked in the library catalog, found a record that looks useful, and located that book on the shelf, the other items AROUND your book will be on the same topic. Score!