The Library of Congress Classification System is used by most academic libraries to organize books on the library shelves. It is an alpha-numeric system. Each item in the library has a label with letters and numbers (together called a call number) which indicates its place on the shelf. The letters indicate a subject area, and the numbers which follow further subdivide the broader topic into more specific categories.
Organized by Subject
Academic libraries organizes items on the shelves by subject. Usually, it's by the first subject listed in a catalog record. This means that books primarily about Mark Twain, will be next to other books about Mark Twain; books primarily about the Civil War, will be next to other books about the Civil War; books about the Gospel of Mark, will be next to other books about the Gospel of Mark, etc. So when you go to the shelf to grab a book, take a look at the books surrounding it - they may be what you're looking for, too!
Reading a Library of Congress Call Number
The first part of a Library of Congress call number is one or two letters. These indicate the subjects. The first letter indicates the broad subject.
The second letter represents a narrower subject and the first number is an even narrow subject.
Let's break down an example:
Extramarital Relations, by Gerhard Neubeck is assigned the call number: HQ806 .N46
H is for social sciences
HQ means family, marriage, and women
HQ806 means adultery
N46 represents the authors last name
There may also be a four digit number after the author, which is the publication year.
So how do you know the order of books on a shelf when their numbers are so similar? Let's look at an example that may help:
These 7 books are in the correct order.