Some people find libraries confusing places. They bigger they are, the more intimidating. I have written about asking reference questions and using the catalog, but once you identify something to read, you must decipher KFI1376.L5 V47 2002 or 346.043096761 M891p. These strings of characters are derived from two different library classification systems. Library classification has two functions. It puts every item in the library close to related items, and it tells patrons exactly where to find what they have found in the catalog.
Libraries are not organized like book stores. Book stores use ordinary language, such as Religion, History, Reference, etc. to categorize books. The system works well enough for people who want to browse a broad subject or who want a particular title and can easily identify the category. It works less well for people who want a narrower topic or who want something that crosses categories.
Whatever you find in a library catalog, someone wrote the description, analyzed the subject, and classified it according to strict rules. Most American libraries use one of two classification systems. Academic libraries usually use the Library of Congress Classification (LCC), for example KFI1376.L5 V47 2002. Public libraries usually use the Dewey Decimal Classification (DDC), for example 346.043096761 M891p.
Both of these systems attempt, in different ways, to classify all of knowledge so that any book (or other material) can be kept with others on the same topic. Library classification systems enable catalogers to define very narrow topics.
A bookstore may have a section for law. So does the library, but within that subject, it has a special place for intellectual property law, and within that, for copyright, trademarks, and patents. At any level of detail, the classification can differentiate between a dictionary, a treatise, or a work intended for non-lawyers. It can also separate United States copyright law from, say, Japanese copyright law.
Library patrons do not need to know the exact meaning of every element of the classification number in order to find what they need. In LCC, the first letter represents the broadest subject, and other letters (if any) narrow it. The following numbers narrow it further. In DCC, the numbers before the decimal point work the same way, proceeding from broadest (the first digit) to narrowest (after the decimal point).
Once the classification number has been assigned, the cataloger adds more information to put all of the books with that classification in alphabetical order by author. (V47 in the LCC example, M891 in the DDC). The LCC number in the first paragraph also adds the year of publication.
Items in a library that use DDC are filed in strict numerical order. Anyone who understands decimal numbers in general should have no trouble finding a particular Dewey number.
LCC numbers may look more complicated, but they are not. First, look for the letter prefix in its ordinary alphabetical sequence, then look for the number. The part after the decimal point, even though it begins with a letter, is a decimal number. That is, .L49 comes before .L5.
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