In 1820 the average person in England wrote only three letters a year.And with good-reason---letters in those days were mailed without any cover paper and could be read by anyone. But William Mulready had an ideal to guarantee privacy---the envelope.
On a visit to France he had noticed that messages from an important person were completely enclosed in a "little paper case...impervious to the peering eyes of the curious."
The ideal of sending letters shielded from curious eyes was an instant success. The volume of mail handled by the British postal service soared beyond anyone's expectations. That ideal came across the Alantic to America and spread until today there are billons of "little paper" envelopes traveling around the world.