[One sample] postcard [in our collection] was mailed on a Saturday in 1919. The message concerns an intended ride to town the next day by electric interurban car. “I wrote you a letter telling you I would be in town Sunday on the quarter to 9 car, but that makes me hurry so much that [I] will come later… on the quarter to 11… meet me…and I will take you to church.”

Today, most Americans forget (if they ever knew) how fast and dependable the U.S. Postal Service once was. In many localities, postcards could be used to set and change appointments, even within the course of a few hours’ time. Streetcars carried mailboxes, which were emptied after every run. In many cities, mail was delivered to homes several times a day.

– from Picturing Illinois: Twentieth Century Postcard Art from Chicago to Cairo by John Jakle and Keith Sculle, p. 127. This completely clarifies, to someone going through antique postcards in 2014, the function and timing of postcards from that era. I’m fairly surprised the authors didn’t mention this much earlier in the book.