Patrick Power Library

Call Numbers

What is a call number?

A call number tells you where to find a book in the library. Basically, it is the book's "address." Call numbers ensure that books on the same subject are shelved near each other.

Each call number begins with one or two letters representing a broad class of knowledge: for example, B = Philosophy, D= World History, BF = Psychology, HM = Sociology, PR = Literature, and so on. These classes are further subdivided by numbers representing more specific topics. Thus, a book about the Roman Empire may have the call number: DG 271 R38 2009.

The call number for a book will be found in the catalogue and printed on the book's spine (or front cover).

Finding Books By Call Number

To find a book on the shelf, think of the call number as you would a street address, and track it down one element at a time. If visiting a friend's new apartment, you would first find the street, then look for the correct building number, and then the apartment number. Call numbers can be tracked in the same way. For example, to find a book with the call number DG 271 R38 2009, you would look for the first element (DG), then the second (271), the third (R38)... until you find the exact spot on the shelf where the book belongs.


The third element in the call number is read as a decimal number (e.g., R.38). This means that the book with the call number DG 271 R38 2009 will come before a book on the shelf with a call number DG 271 R5 2008, because .38 is a smaller number than .5.

Example: The below call numbers are arranged in the order in which they would appear on the shelf:

call numbers on shelf

For more help, take a look at our list of call number shelf locationsYou can also view our online tutorial about reading call numbers and finding books on the shelves.

If you have any difficulty finding a book on the shelves, please visit the Research Help Desk, located on the 1st floor of the Library.