From the time humans first began amassing a culture's written records into libraries, they have attempted to arrange records in some reasonable order. With today's massive collections, classification systems have become quite extensive and involved. There are two classification systems:

1. Dewey Decimal Classification
2. Library of Congress Classification

Dewey Decimal Classification

This was, for many years, the most common classification system used by libraries, and it is still used by many public and school libraries today.

Dewey developed his system predicated on the idea that the major areas of knowledge could be divided into ten major divisions of three-digit numbers (000, 100, 200, etc. to 900). For example, all of the numbers between 500 and 599 deal with some aspect of a single subject ­ pure science. Each of these divisions is, in turn, divided into ten subdivisions (510, 520, 530, etc.). Any book, then, with the exception of fiction, is classified with a number between 001 and 999.

Since greater refinement is often necessary, a further series of decimal numbers is employed to increase the distinction in subject matter. At times, the use of decimal numbers causes difficulty for library users and perpetuates the myth that there is some mystery that only librarians can fathom in locating a classified book. However, once students understand the use of decimals, there is no mystery to the sequence.

Every Dewey number has at least three digits. They are shelved in a numerical sequence. Thus, 001 precedes 099, which precedes 100.

Any digits, which follow the first three, are decimal numbers. Thus, in the classification numbers 910.305 and 910.35, the digits following the decimals (.305 and .35) are arranged decimally, rather than numerically. Thus:

Library of Congress Classification

Since the Library of Congress utilizes most of the letters of the alphabet for major divisions, a second letter for subdivisions, and a series of numbers for further refinement, the possibilities for expansion are much greater than with the Dewey Classification system

Students familiar with the decimal classification are sometimes confused by the use of letters. Overall, students tend to find the Library of Congress Classification a simpler system, once they discover that it, too, is merely a sequence utilizing letters, as well as numbers.

The numbers which follow the letters in the Library of Congress Classification (1 through 9999) are shelved numerically. Thus, the following five books are shelved from left to right numerically:

Occasionally, a decimal number appears in a Library of Congress Classification number. It is, of course, shelved decimally. Thus:

D 8410 precedes

D 8410.12 which precedes

D 8410.5 and before

D 8411

The Library of Congress Subject Headings (LCSH) is a set of four big red books, usually shelved in the Reference collection of most libraries. It is the system established by the United States Library of Congress, and used by most library catalogs.

The LCSH involves the concept of controlled vocabulary. In the printed publications environment, when searching for information from printed indexes or card catalogs, the searcher's subject terms must match with those of the indexer. For example, if the searcher uses the term death penalty and gets no result, it is because the indexer uses the term capital punishment. Often times, there may be cross-references leading the searcher to the correct terms to use. Hence, there must be a system whereby standard subject headings are assigned to concepts, a sort of common language.

The LCSH was created and it is a controlled subject heading lists upon which indexers and catalogers refer to when doing their tasks of indexing and cataloging. It is a controlled vocabulary environment with predetermined subject headings which does not always brings results.

The LCSH is updated regularly to reflect changes in languages and cultures. For example, subject headings reflecting changes in American life and culture were recently added: AIDS, Internet, and cyberpunk.

Subject headings are listed in boldface type, e.g., Motorboats, Agricultural machinery, Drugs. A heading may be followed by the note (May Subd Geog), indicating that the heading may be subdivided by geographic locations. Subject headings may consist of one word or several words. A one-word heading is often a noun, e.g., Motorboats; two-word headings are usually composed of an adjective and a noun, e.g. Agricultural machinery.

Subject headings often include some abbreviations denoting hierarchical relationship between various topics. You need to become familiar with these abbreviations. Refer to the introductory chapter at the beginning of the volume to learn how to interpret the entries. This is a good habit to follow when using reference sources in the library: to always start first with the how-to-use section of every reference sources; they all include an instructional preface or introduction.

Here is one entry example in the Library of Congress Subject Headings:

Agricultural machinery (May Subd Geog)
[S671-S760]
UF Agriculture - Equipment and supplies
Crops - Machinery
Farm machinery
BT Machinery
RT Farm equipment
Farm mechanization
Machine-tractor stations
SA subdivision Machinery under names of
crops, e.g. Corn - Machinery
NT Agricultural implements
Agricultural instruments

In the example above for the subject heading Agricultural machinery, we see that it can be subdivided geographically as indicated by the note (May Subd Geog). Furthermore, [S671-S760] indicates the class number to use for cataloging books in this subject. The following abbreviations are read as thus:

UF indicates that the heading Agricultural machinery is Used For those listed after UF, i.e.,Agriculture - Equipment and supplies, Crops - Machinery, Farm machinery

BT indicates that the Broader Topic for Agricultural machinery is Machinery

RT indicates that the Related Topic for Agricultural machinery is Farm equipment, Farm mechanization, and Machine-tractor stations

SA refers to subjects that user could See Also

NT indicates that the Narrower Topic for Agricultural machinery is Agricultural implements, Agricultural instruments

Let us look at one example of a hierarchical relationship:

Vehicles
BT Transportation
NT Motor vehicles
Motor vehicles
BT Vehicles
NT Trucks
Trucks
BT Motor vehicles
NT Dump trucks
Dump trucks
BT Trucks

The hierarchy tree above provides a clearly defined organization of the big family tree which is Transportation into branches, i.e., Vehicles and Motor vehicles, and twigs, i.e., Trucks and Dump trucks. This example shows that users could follow the hierarchy from the narrowest topic Dump trucks to the broadest topic Transportation.