First US Aerogram
On April 29, 1947, the US issued its first aerogram, then called an air letter sheet.
In the early days of the mail, letter sheets were the common way to send letters. Messages were written on paper that was folded and sealed. Some postal authorities would charge by the amount of paper used, so using an envelope would make mailing a letter more expensive. As these laws changed and envelopes became the common practice, letter sheets fell out of favor but were still used on occasion in the 1900s.