A call number is simply a number, or letter-number combination, used to identify items in the library. There are various classification systems employed by libraries throughout the world. The Hagerty Library uses the Library of Congress Library Classification System. At the Health Sciences Libraries, the National Library of Medicine classification system is utilized. The codes make it possible to identify and retrieve individual items in the library's collection.

A book may have the same call number at Drexel as it does at the University of Pennsylvania or UCLA, etc. However, no two books in any one library should have the same call number- a book is identified by a unique number. The book is identified rather than the specific shelf location in order to accommodate growth in the Library's collection. No matter how many new books are purchased and no matter how often books are shifted to make room on the shelves, one can always find individual items using the call number as a guide.

Why not just give each book a number as they are purchased and place them on the shelves? This would certainly identify each item as unique, and all the new books would be easy to find but most people need books about a particular topic. Both the Library of Congress system and the National Library of Medicine system are based on subject. For instance, say you would like to get a dog for a pet and want to learn more to determine what type of dog would be best for you. Call numbers are assigned to books based on what the book is about. All books about pets have similar numbers because the subject is pets. Books about cats or dogs or fish are further divided into like sets. Dog books about breeds are placed with like books while books about training dogs are place d nearby but with other books on the specific subject of training.

Library of Congress Classification System Example (Hagerty Library) Here is the name of a book in the Hagerty Library collection, followed by its location and call number : Building large knowledge-based systems: representation and inference in the Cyc project / Douglas B. Lenat, R.V. Guha. Hagerty Books Q335 .L45 1989   The letter Q represents the class or subject SCIENCE. The class is broken into subclasses: Q =General science; QA =Mathematics; QB =Astronomy; QB =Physics, etc. So, Q in the example above tells us that this is a science book. Since it is just Q , not QA , etc., it is General Science.

National Library of Medicine Classification System Example (Health Sciences Libraries) Here is the name of a book in the Health Sciences Library collection, followed by its location and call number : Manual of medical-surgical nursing care: nursing interventions and collaborative management / Pamela L. Swearingen, special project editor; Dennis G. Ross. HAHN Book Collection WY 49 M2944 1999 The letter W= Clinical Medicine. The class is broken down into subclasses, and WY=Nursing.

Okay, I get the concept of the letters. What about the numbers?

Library of Congress Classification System Example (Hagerty Library) The Library of Congress assigns more specific subjects areas by appending numbers to the letter combinations. The subclass Q representing general science is further broken down this way: Q1-295: General Q300-90: Cybernetics Q350-90: Information theory Therefore, the title in this example falls into the category of cybernetics. You might also note that information theory is a subset of cybernetics.

National Library of Medicine Classification System Example (Health Sciences Libraries) WY 1-100.5: General WY 101-145: Special fields in nursing WY 150-164: Nursing techniques in special fields of medicine WY 191-200: Other nursing services WY 300: By country Therefore, the title in this example falls into the category of general works in nursing.

Wait! There must be thousands of books about either topic. You said each book gets a unique number. The second part of the call number represents the individual item. In the above Library of Congress Classification example, the whole number is followed by a decimal, then the letter-number combination L45. Here the L represents the first letter of the last name of the first author, Lenat , then using a coding sequence referred to as a "Cutter number," the second and third letters in Lanet's name are assigned numbers. The National Library of Medicine's Classification system works in the same fashion!

Each letter represents a very broad subject area. The classes are broken into subclasses with the addition of letters and numbers. The tables below show each letter and the broad subject area it covers.
 

Library of Congress Classification System (Hagerty Library)
A:	GENERAL WORKS	G:	GEOGRAPHY. ANTHROPOLOGY. RECREATION	N:	FINE ARTS	U:	MILITARY SCIENCE
B:	PHILOSOPHY. PSYCHOLOGY. RELIGION	H:	SOCIAL SCIENCES	P:	LANGUAGE AND LITERATURE	V:	NAVAL SCIENCE
C:	AUXILIARY SCIENCES OF HISTORY	J:	POLITICAL SCIENCE	Q:	SCIENCE	W:	Not assigned
D:	HISTORY: GENERAL AND OLD WORLD	K:	LAW	R:	MEDICINE	X:	Not assigned
E:	HISTORY: AMERICAS	L:	EDUCATION	S:	AGRICULTURE	Y:	Not assigned
F:	HISTORY: AMERICAS	M:	MUSIC AND BOOKS ON MUSIC	T:	TECHNOLOGY	Z:	LIBRARY SCIENCE

Note the absence of I and O , letters that can be easily confused with numbers .
  

National Library of Medicine Classification System (Health Sciences Libraries)
QS:	HUMAN ANATOMY	WA:	PUBLIC HEALTH	WI:	DIGESTIVE SYSTEM	WQ:	OSTETRICS
QT:	PHYSIOLOGY	WB:	PRACTICE OF MEDICINE	WJ:	UROGENITAL SYSTEM	WR:	DERMATOLOGY
QU:	BIOCHEMISTRY	WC:	COMMUNICABLE DISEASES	WK:	ENDOCRINE SYSTEM	WS:	PEDIATRICS
QV:	PHARMACOLOGY	WD:	NUTRITION DISORDERS, METABOLIC DISEASES, ETC.	WL:	NERVOUS SYSTEM	WT:	GERIATRICS, CHRONIC DISEASE
QW:	MICROBIOLOGY & IMMUNOLOGY	WE:	MUSCULOSKELETAL SYSTEM	WM:	PSYCHIATRY	WU:	DENTISTRY, ORAL SURGERY
QY:	CLINICAL PATHOLOGY	WF:	RESPIRATORY SYSTEM	WN:	RADIOLOGY, DIAGNOSTIC IMAGING	WX:	HOSPITALS & OTHER HEALTH FACILITIES
QZ:	PATHOLOGY	WG:	CARDIOVASCULAR SYSTEM	WO:	SURGERY	WY:	NURSING
W:	HEALTH PROFESSIONS	WH:	HEMIC & LYMPHATIC SYSTEMS	WP:	GYNECOLOGY	WZ:	HISTORY OF MEDICINE