What subjects do the call letters cover?
Subclass C Auxiliary Sciences of History (General).
Subclass CB History of Civilization.
Subclass CC Archaeology.
Subclass CD Diplomatics. Archives. Seals.
Subclass CE Technical Chronology. Calendar.
Subclass CJ Numismatics.
Subclass CN Inscriptions. Epigraphy.
Subclass CR Heraldry.
Subclass CS Genealogy.
Subclass CT Biography.