The Putnam Classification System is a library classification system developed by George Herbert Putnam.
He developed a handwritten system of classification, dividing the books into categories and subcategories.
The letters were written on the spine of the books in white paint.
The system is still in use in parts of the Hennepin County Library.
[1] Putnam went on to become the Librarian of Congress, where he retooled his system and developed the current Library of Congress Classification system.