A call number is a string of letters and numbers given to a physical book to designate its location on the shelf, as well as its subject classification. The University of Iowa Libraries uses the Library of Congress Classification system that arranges books into 21 broad general categories, with each category represented by a single letter of the alphabet. To learn more, visit the Library of Congress' website: https://www.loc.gov/catdir/cpso/lcc.html.

Below is an example of a call number beginning with the letters HQ. H tells us that the book is in the broad category of social sciences. The first two letters, HQ tells us the book is in the sub-category of "Family, Marriage, and Women."