Library

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:

HD2755.5.K575

Structure of LC Call Numbers

Divisions and Subdivisions

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.

Subject

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 "Corporations".

Cutter Number

The final letter/number combination after the last decimal point is called the "Cutter Number".  It usually reflects the name of the author.

In our example, the author's last name is Klein, so the cutter number begins with 'K'.

The actual numbers after this letter are assigned so that all books with a given call number file in alphabetical order by author.

Order of LC Call Numbers

Locating an item on the shelf can sometimes be tricky, because of how LC call numbers are filed. Keep the following in mind and you should have no problems.

1. The initial letter(s) file in alphabetic order:

B... BA... BC... BD... C... CA... CB... D...

2. The first numbers (before the first decimal point) are treated as a whole number, not as individual numbers and these then file in numerical order:

Correct: QA29.A6 ... QA31.A6 ... QA32.A6 ... QA311.A6 ...
Incorrect: QA29.A6 ...QA31.A6 ...QA311.A6 ...QA32.A6 ...

3. If there are numbers after the decimal point, these are treated as decimals - not as whole numbers:

Correct: HD1995.12.A6 ... HD1995.5.A6 ... HD1995.65.A6 ... HD1995.7.A6
Incorrect: HD1995.5.A6 ... HD1995.7.A6... HD1995.12.A6 ... HD1995.65.A6

4. The letter following the decimal point files in alphabetical order.

5. The final numbers following the final letter are treated as a decimal, not as a whole number:

Correct: HD234.2.A60... HD234.2.A62... HD234.2.A622.. HD234.2.A65...
Incorrect: HD234.2.A60... HD234.2.A62... HD234.2.A65... HD234.2.A622..