APA (7th ed.) Citation

United States. Post Office Department. (1921). Postal Union transit statistics (May 1-28, inclusive, 1921). GPO.

Chicago Style (17th ed.) Citation

United States. Post Office Department. Postal Union Transit Statistics (May 1-28, Inclusive, 1921). [Washington]: GPO, 1921.

MLA (9th ed.) Citation

United States. Post Office Department. Postal Union Transit Statistics (May 1-28, Inclusive, 1921). GPO, 1921.

Warning: These citations may not always be 100% accurate.