Let's go over the different parts of what makes up a complete LOC call number. There are two main parts: Class number and Item number.
Class number is broken into two parts: Main class and Subclass. The Main class is the first (or only) letter in the line. The Subclass is the second letter in the line.
Item number is broken into two parts: Divisions and Publication Year. The Divisions constitute of numerous subdivisions that specifically categorize the item, depending on the alphanumeric combination. The Publication year is when the item was published, and they always are at the end of an LOC call number.
Here are some links to learn how to read LOC call numbers. If it is still confusing, find your friendly Lindsey librarian and we will help you!
These are the major classes (or categories) of the Library of Congress system. There are 21 total. Check out the other pages for more information on their subclasses.
A: General works
B: Philosophy, Psychology, Religion
C: Auxiliary Sciences of History
D: World History
E: History of the Americas
F: History of the Americas
G: Geography, Anthropology, Recreation
H: Social Sciences
J: Political Sciences
M: Music and Books on music
N: Fine Arts
P: Language and Literature
U: Military Science
V: Naval Science
Z: Bibliography, Library Science, Information Resources (general)
The Subclasses are the subcategories under each Main class. They are represented as a letter (A-Z). There is no particular order for the subclasses as long as the major subcategories are recorded.
Divisions are the subcategories of Subclasses using an alphanumeric combination to further classify subjects. They use a representation called Cutter numbers, which further breakdown subcategories. It is helpful when, for example, there are authors with the same first and last name, or same last name. Same for items with similar or the same titles. Divisions are very important because they make sure everything item has a unique LCC call number.
Did you notice how not the entire English alphabet was used? The letters I, O, W, X, and Y are not assigned to any major categories. While there is no guidance regarding the letters I and O, it is assumed these letters can be easily confused for the numbers 1 and 0, respectively. The letter W probably does not have a category because the letter W represents Medicine and Related Subjects in the National Library of Medicine Classification; this also includes the subcategories QS-QZ, which represent Preclinical Sciences in the same classification.
However, the letters X and Y simply have not been assigned any categories, thus allowing for possible expansion should new major topics arise. The same goes for I, O, W, and QS-QZ.