Why does Widener Library use more than one call number system?

Many older libraries and archival collections, including some at Harvard, use more than one classification system.

What is a library classification system? 

A library classification system is a system of organizing library materials by topic so that similar books are shelved together. Classification systems use numeric and alphanumeric codes to create call numbers according to the system.

• In the US, most school and public libraries use the Dewey Decimal System, while many college and university libraries use Library of Congress. 
• Many specialty collections choose call number systems according to the size and subject scope of their materials, and several Harvard Library collections use multiple call number systems, some historic and some more current.

What systems does Widener use?

Widener began with a system we now call "Old Widener" in about 1915. Old Widener was not as flexible and expansive as some newer systems, so to accommodate the ever-growing collection and to be more consistent with other university research libraries of its kind, Widener adopted the Library of Congress system in the late 1970s.

• It was not logistically feasible to convert the Old Widener numbers to Library of Congress numbers, so Widener now has two systems.
• All materials acquired before the change are still shelved under Old Widener numbers.
• All materials acquired later are shelved under Library of Congress.
  • All books acquired for Widener after the change are shelved under Library of Congress, regardless of publication date.