A Call Number is the "address" of a book that is written on its edge. The Library of Congress Classification System uses a combination of letters and numbers to arrange materials on library shelves by subject.


Each side of an aisle of the bookshelves has two call numbers on white paper on the end to represent a range that runs from upper left to lower right of the aisle. The first code is the call number of the first book on the upper left of the aisle, the last is that of the last book on the lower right at the far end of the aisle. Each vertical section of the metal shelving runs a portion of this range, and then starts anew at the top left, like the pages within a book.