The classification system used to organize the materials in the WDH Carolina Research Center is different from the Dewey Decimals used in the rest of the PCPL system. It is primarily geographical, but it isn't limited to just geographical information. Since it is so different, it takes a bit of use to grow comfortable with it. There's a poster of the system in the building for quick reference, and you can also jump to an on-line quick reference.
For a full in-depth discussion, read below.
The classification starts with a two-letter code assigning material to a broad category. The most common categories are US states, represented by two-letter postal codes. Other categories cover non-geographically specific topics. They are: general information (especially on genealogy resources and how-to); biographies; church histories; historic preservation; Ethnic and Emigration/Imigration; Family Histories; Genealogies not specific to a US state; History; a Miscelaneous category, for what ever doesn't fit anywhere else.
Next comes a three-letter code decribing the content of the material. Again, most common is a geographical reference: a three-letter abbreviation for a county within a US state. Other codes are used for non-geographically-specific materials, or materials which refer to more than one county.
The next level of classification is a numeric designator used to specify the topic of material. These numeric designators are used with all of the category and content (two and three letter) codes, and mean the same thing regardless of which category/content they're used with. For example, the designator ".3" always means public records, whether in genealogy, biography, history, or whatever.
After category, content, and topic comes a three-letter abbreviation of the last name of the author, editor, or compiler of the material.
Materials are divided into categories using a two-letter code.
Each of these will be modified by content (three letter) code and topic numeric designator to help make the classification very specific to a geographic location (or other content) and topic within the category. Note that the content code and topic designator always mean the same thing, regardless of which top level category they apply to.
The two exceptions to the "category - content - topic" classification format are the biographies and family histories, as mentioned in the table above.
The Content classifications are assigned three-letter codes which can be geographical or topical.
Of these, only IND is currently also a country code (India). It should be pretty clear from the category and/or topic which content is intended.
AAA and ZZZ deserve further clarification:
Topic designators specify what information the material holds. Topic designators always have the same meanings across all categories and content. They are appended to the content code with a period ("."). The first digit after the period indicates a broader area, and the second digit specifies more completly within that area. In rare cases a third digit may be required or used for clarity.
The general form of the classification is a two-letter category code, three-letter content code, the topic 2 or 3 digit numerical designator, and the first three letters of the last name of the author, editor, or compiler. Three exceptions:
Two-letter codes may be either the USPS state/territory code, or one of:
Three-letter codes are either geographical (county, country, continent) or one of:
PCPL adapted this classification system from the Rowan County Public Library system, who in turned adapted it from Arlington, TX public libraries. Many thanks to them for creating a classification well suited to genealogical and local history. The original (scholarly) paper describing the creation of the sytem may be read here (1 MB PDF file): Arranging Roots: Classification and Subject Headings for Genealogical Collections.