Cita APA (7th ed.)
United States Postal Service. The Postal Service Guide to U.S. Stamps: Updated Stamp Values (Vigésima octava edición.).
Cita Chicago (17th ed.)
United States Postal Service. The Postal Service Guide to U.S. Stamps: Updated Stamp Values. Vigésima octava edición.
Cita MLA (9th ed.)
United States Postal Service. The Postal Service Guide to U.S. Stamps: Updated Stamp Values. Vigésima octava edición.
Atenció: Aquestes cites poden no estar 100% correctes.