APA (7th ed.) Zitazioa
United States Postal Service. The Postal Service Guide to U.S. Stamps: Updated Stamp Values (Vigésima octava edición.).
Chicago Style (17th ed.) Zitazioa
United States Postal Service. The Postal Service Guide to U.S. Stamps: Updated Stamp Values. Vigésima octava edición.
MLA (9th ed.) Zitazioa
United States Postal Service. The Postal Service Guide to U.S. Stamps: Updated Stamp Values. Vigésima octava edición.
Kontuz: berrikusi erreferentzia hauek erabili aurretik.