An Aerogram, Aérogramme, Air Letter or Airletter is a thin lightweight piece of foldable and gummed paper for writing a letter for transit via airmail, in which the letter and envelope are one and the same. Most postal administrations forbid enclosures in these light letters, which are usually sent abroad at a preferential rate. Printed warnings existed to say that an enclosure would cause the mail to go at the higher letter rate.