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