The Cardan grille is a method of writing secret messages using a grid. History This technique was used in ancient China. In 1550, Girolamo Cardano (1501–1576), known in French as , proposed a simple grid for writing hidden messages. He intended to cloak his messages inside an ordinary letter so that the whole would not appear to be a cipher at all. Such a disguised message is considered to be an example of steganography, which is a sub-branch of general cryptography. But the name Cardan was applied to grilles that may not have been Cardan's invention, and, so, Cardan is a generic name for cardboard grille ciphers. Cardinal Richelieu (1585–1642) was reputed to be fond of the Cardan grille and to have used it in both private and diplomatic correspondence. Educated men in 17th century Europe were familiar with word games in literature, including acrostics, anagrams, and ciphers. Although the original Cardan grilles were little used by the end of the 17th century, they still appeared occasionally in the form of masked letters and as literary curiosities. George Gordon Byron, for example, claimed to have written Cardan-grille verse – but as a demonstration of verbal skill rather than a serious cipher. Alternative grilles possess apertures for single letters only and can be used quickly. Messages are filled out with a jumble of letters or numbers and are clearly cryptograms whereas Cardano intended to create steganograms. These single-letter grilles may be named after Cardano, but they are also called simply cardboard ciphers. A further variation is a turning grille or trellis, based on the chess board, which was used in the latter 16th century. The turning grille reappeared in a more sophisticated form at the end of the 19th century; but, by this time, any connection with Cardano was in name alone. Construction A Cardan grille is made from a sheet of fairly rigid paper or parchment, or from thin metal. The paper is ruled to represent lines of handwriting and rectangular areas are cut out at arbitrary intervals between these lines. An encipherer places the grille on a sheet of paper and writes his message in the rectangular apertures, some of which might allow a single letter, a syllable, or a whole word. Then, removing the grille, the fragments are filled out to create a note or letter that disguises the true message. Cardano suggested drafting the text three times