The Billings Classification is a subject classification developed by John Shaw Billings in 1899. It was not an abstract experiment in the mapping of human knowledge, but rather a system devised specifically for the optimum arrangement of the Library’s collections. Dr. Billings envisioned a layout for the building, and while his classification scheme was in fact a division of human intellectual achievement into various subject areas designated by letter codes varying in number (usually three), it was fashioned in such a way as to maximize access to the unique holdings of the Library. The planned physical space—how things would function—all played a role in the construction of his classification. The majority of the Billings classmarks are not longer in use, as NYPL began using a fixed order system in the 1950s.