- HwoCd = HWO's Classifications - DDC = DeweyDecimalClassification system
- HwoCl = HWO's Classifications - LibraryOfCongress system
- HwCh? = HWO's personal classifications system(s)
- A General works
- B Philosophy; psychology; religion
- C Auxiliary sciences of history
- D History: General and Old World
- E-F History: America
- G Geography; anthropology; recreation
- H Social sciences
- J Political science
- K Law
- L Education
- M Music and books on music
- N Fine arts
- P Languages and literature
- Q Science
- R Medicine
- S Agriculture
- T Technology
- U Military science
- V Naval science
- Z Library science
