We use the term ‘back of the envelope’ to capture the notion of jotting down an idea or plan on the spur of the moment. The film director Roberto Rossellini clearly made use of his, but it’s quite a quirk never to open letters, which of course would make it impossible to answer them.

Those letters were in the wastepaper basket, and so were probably destroyed.  But imagine the title of a book: Unopened Letters to Roberto.

‘Okay, he was thinking, and one day he said, ‘Just give me one of those old envelopes, will you?’  I handed one over.  We had dozens.  He never opened envelopes.  He never opened a letter and he never answered one.  The wastepaper basket was full of unopened letters.  So now he was scribbling on the back of the envelope.’

Source: Ingrid Bergman: My Story, Ingrid Bergman & Alan Burgess (London: Sphere Books, 1980), p. 339