Libraries use a classification system to organize the books on the shelves. This system uses letters and numbers (call numbers) to arrange books so that books on the same topic are grouped together on the shelves.
You can find the call number at the bottom of the spine of a book or in the details of an item found in the Library catalog:
Ask a librarian or staff member for assistance if you need help finding a book. Additionally Portland State University Library made a useful video on how to find a book using its call number.