Most items in the library are shelved by Library of Congress (LC) call numbers.
As an example, here's the call number for the book No logo: taking aim at the brand bullies by Naomi Klein:
The first letter of a call number represents one of the 21 major divisions of the Library of Congress system. In our example, 'H' stands for "Social Sciences".
The second letter, if present, represents a subdivision. In our example, "D" is for "Industry, Labour, & Land Use".
For most subject areas, books that are given the single letter are usually of a general nature for that subject area. For example, the call number for the book Action and interpretation: studies in the philosophy of the social sciences is H61.A39.
The first set of numbers in a call number help to define the item's subject.
In our example, '2755.5' falls in the range of HD2709-HD2930.7 which deals with