Not long ago you could buy special air-mail letters. These were thin blue paper, with an envelope front printed on one side and a gummed trip on the other. You'

Not long ago you could buy special air-mail letters. These were thin blue paper, with an envelope front printed on one side and a gummed trip on the other. You'd write your letter on the back, fold them up and stick them down, and post at reduced rates (because they were guaranteed to be light, having no enclosures). I haven't seen one for ages. I hope they haven't vanished completely; they were a really good idea...