~~~ years ago.

If you tucked the flap in the envelope it was a penny less to mail a letter-size envelope.

I could never understand how that didn't cause problems with other envelopes sliding in to the opening and messing up the process.

Not sure about the clasp envelopes or the ones with the button and string, but I do know that by not sealing those, one could reuse the envelope... maybe it was part of the war effort to conserve (WWII). I don't know.