Dewey Decimal Classification (DDC), created 1876. Still used by many public and school libraries, but largely abandoned by academic libraries in the late 20th century.
Established in 1897, completed into modern form ~1969. By the late 20th century, it became the standard for academic libraries and large public libraries in the United States.
As we talk about the library's services, databases, and the general organization of information, consider: