Books in the library have call numbers, which are like addresses for their location on the shelf.

Let's say you were looking for the book Perfectly legal : the covert campaign to rig our tax system to benefit the super rich. By searching for the title "perfectly legal" in the library catalog, you would find the call number.

The call number for Perfectly Legal is 336.2 CAY.
Besides telling you exactly where the book is located in the Library, what does the call number tell you about the book?

336		The 300's represent the broad category "social science." 336, in particular, is used for books about public finance.
.2		Numbers after the decimal represent a more specific subject area. In this case, taxation.
CAY		The letters are usually the first three letters of the author's last name