More to explore:
Letters and Envelopes / camp griffin
Letters from a Civil War soldier, home - I'm not sure if the 'covers' as they were called were army issue, but I've now seen quite a few from soldiers to home that look like this.
....so sad that the youngs do not know the happiness of receiving a handwritten letter!!!!
Old Air Mail Envelopes Part 1 - Free Hi-Res Images. Images are approximately 6000x3500 pixels each. Nice.
aaahh . . . the charm of a beautifully written letter with unique stamps
Diy: free printable mini envelope template
Airmail. And the envelopes where you wrote on the inside that were really thin to reduce the weight of the letter.
reversed junk mail envelopes
Gerry Jackson Kerdok by WashCalligraphersGuild, via Flickr
Patterned envelopes! Find any envelope pattern, and head straight to patterned paper. Such a pretty spring thing.